diff --git a/src/simulator/crates/mc-core/src/content.rs b/src/simulator/crates/mc-core/src/content.rs
new file mode 100644
index 00000000..72d5c11d
--- /dev/null
+++ b/src/simulator/crates/mc-core/src/content.rs
@@ -0,0 +1,218 @@
+//! Host-fed content registry — the single seam through which simulation crates
+//! read canonical JSON game content (Rail-2).
+//!
+//! # Why this exists (p3-28, "boot-config DRY")
+//!
+//! Content reaches the simulation two ways and they must never drift:
+//! - **In-game** (Godot drives): the GDScript `DataLoader` reads packs at
+//!   runtime and the host injects the bytes across FFI.
+//! - **Headless / WASM / tests** (Rust drives alone): no `DataLoader`, so Rust
+//!   falls back to a compile-time `include_str!` copy.
+//!
+//! Before this registry, every content-owning crate rolled its *own*
+//! `OnceLock` + a fragile `../../../../../public/...` relative `include_str!`.
+//! That scattered the embedded fallbacks across ~7 modules and made the
+//! two-path divergence (a hardcode drifting from the JSON) easy to reintroduce.
+//!
+//! This module collapses that into one seam:
+//! - **Embedded defaults** live in exactly ONE place — the [`EMBEDDED`] table
+//!   below (the only `include_str!` content sites for these packs). This is the
+//!   headless / WASM / test fallback. It is filesystem-free, so it is
+//!   WASM-safe and GDExtension-safe.
+//! - **Host injection** — [`register`] lets the host (Godot from `res://`
+//!   bytes, the web guide from `fetch`ed bytes) override a pack at engine init.
+//!   A file-reading `boot_from_resources(path)` would be *wrong* here: the
+//!   shared sim compiles to WASM, which has no filesystem. The host feeds
+//!   bytes; the registry never touches the filesystem.
+//! - **Consumers** call [`get`] for the raw JSON of a pack, keyed by a stable
+//!   [`ContentKey`], and keep their own typed `OnceLock` parse cache as before
+//!   — only the *source of the bytes* moves here.
+//!
+//! Hosts are expected to inject (if at all) at engine init, **before** any
+//! consumer first reads a pack: a consumer's typed cache is a `OnceLock`, so
+//! the first read wins for the process lifetime. Injecting after a pack has
+//! been read has no effect on that pack's already-parsed cache — same lifetime
+//! contract the per-crate `OnceLock`s always had.
+
+use std::borrow::Cow;
+use std::collections::HashMap;
+use std::sync::{OnceLock, RwLock};
+
+/// A canonical content pack the simulation reads. The string value is the
+/// stable registry key the host injects against and consumers read by.
+///
+/// Adding a pack: add a variant, its key, and its embedded `include_str!`
+/// default in [`EMBEDDED`]. That keeps the embedded-fallback list in one place.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum ContentKey {
+    /// `public/resources/promotions/promotions.json` — promotion XP tuning.
+    Promotions,
+    /// `public/games/age-of-dwarves/data/comms.json` — communications config.
+    Comms,
+    /// `public/games/age-of-dwarves/data/couriers.json` — courier / severable infra.
+    Couriers,
+    /// `public/resources/ecology/encounter_rates.json` — authored encounter rates.
+    EncounterRates,
+    /// `public/resources/ecology/fauna/lair_combat_modes.json` — lair-siege config.
+    LairCombatModes,
+    /// `public/games/age-of-dwarves/data/score.json` — score / award weights.
+    ScoreWeights,
+    /// `public/resources/resources.json` — resource catalog (web guide).
+    Resources,
+}
+
+impl ContentKey {
+    /// The stable string key. Hosts inject against this; the Rail-2 verify gate
+    /// keys its manifest on the embedded-default file path, not on this.
+    #[must_use]
+    pub const fn as_str(self) -> &'static str {
+        match self {
+            ContentKey::Promotions => "promotions",
+            ContentKey::Comms => "comms",
+            ContentKey::Couriers => "couriers",
+            ContentKey::EncounterRates => "encounter_rates",
+            ContentKey::LairCombatModes => "lair_combat_modes",
+            ContentKey::ScoreWeights => "score_weights",
+            ContentKey::Resources => "resources",
+        }
+    }
+}
+
+/// The embedded fallback for each pack — the ONLY `include_str!` content sites
+/// for these packs. Filesystem-free (WASM- and GDExtension-safe). The host may
+/// override any of these at init via [`register`].
+///
+/// Paths are relative to this file (`crates/mc-core/src/content.rs`); five `..`
+/// reach the repo root: `content.rs` → `src` → `mc-core` → `crates` →
+/// `simulator` → `src` → repo root.
+const EMBEDDED: &[(ContentKey, &str)] = &[
+    (
+        ContentKey::Promotions,
+        include_str!("../../../../../public/resources/promotions/promotions.json"),
+    ),
+    (
+        ContentKey::Comms,
+        include_str!("../../../../../public/games/age-of-dwarves/data/comms.json"),
+    ),
+    (
+        ContentKey::Couriers,
+        include_str!("../../../../../public/games/age-of-dwarves/data/couriers.json"),
+    ),
+    (
+        ContentKey::EncounterRates,
+        include_str!("../../../../../public/resources/ecology/encounter_rates.json"),
+    ),
+    (
+        ContentKey::LairCombatModes,
+        include_str!("../../../../../public/resources/ecology/fauna/lair_combat_modes.json"),
+    ),
+    (
+        ContentKey::ScoreWeights,
+        include_str!("../../../../../public/games/age-of-dwarves/data/score.json"),
+    ),
+    (
+        ContentKey::Resources,
+        include_str!("../../../../../public/resources/resources.json"),
+    ),
+];
+
+/// Look up the embedded fallback for a key. Always present — the table is the
+/// compile-time source of truth and is verified non-empty by tests.
+fn embedded(key: ContentKey) -> &'static str {
+    EMBEDDED
+        .iter()
+        .find(|(k, _)| *k == key)
+        .map(|(_, v)| *v)
+        .expect("every ContentKey has an embedded default")
+}
+
+/// Host-injected overrides, keyed by [`ContentKey::as_str`]. Empty until a host
+/// calls [`register`]. `RwLock` because reads vastly outnumber the init-time writes.
+fn overrides() -> &'static RwLock<HashMap<&'static str, String>> {
+    static OVERRIDES: OnceLock<RwLock<HashMap<&'static str, String>>> = OnceLock::new();
+    OVERRIDES.get_or_init(|| RwLock::new(HashMap::new()))
+}
+
+/// Inject host-supplied bytes for a content pack, overriding the embedded
+/// default. Call at engine init, before any consumer first reads the pack
+/// (consumer caches are `OnceLock` — first read wins).
+///
+/// Godot passes `res://` bytes here; the web guide passes `fetch`ed bytes. No
+/// filesystem access happens in this crate — that is the host's job, keeping
+/// the shared sim WASM-safe.
+pub fn register(key: ContentKey, json: impl Into<String>) {
+    let mut map = overrides()
+        .write()
+        .expect("content override lock poisoned");
+    map.insert(key.as_str(), json.into());
+}
+
+/// Clear all host overrides, reverting every pack to its embedded default.
+/// Primarily for tests that exercise injection without leaking across cases.
+pub fn clear_overrides() {
+    overrides()
+        .write()
+        .expect("content override lock poisoned")
+        .clear();
+}
+
+/// Raw JSON for a content pack: the host-injected override if present, else the
+/// embedded default. The returned `Cow` borrows the embedded `&'static str` in
+/// the common (no-override) path, allocating only when an override exists.
+#[must_use]
+pub fn get(key: ContentKey) -> Cow<'static, str> {
+    let map = overrides().read().expect("content override lock poisoned");
+    match map.get(key.as_str()) {
+        Some(s) => Cow::Owned(s.clone()),
+        None => Cow::Borrowed(embedded(key)),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn every_key_has_an_embedded_default_that_parses() {
+        for key in [
+            ContentKey::Promotions,
+            ContentKey::Comms,
+            ContentKey::Couriers,
+            ContentKey::EncounterRates,
+            ContentKey::LairCombatModes,
+            ContentKey::ScoreWeights,
+            ContentKey::Resources,
+        ] {
+            let raw = embedded(key);
+            assert!(!raw.is_empty(), "{:?} embedded default is empty", key);
+            serde_json::from_str::<serde_json::Value>(raw)
+                .unwrap_or_else(|e| panic!("{:?} embedded default must be valid JSON: {e}", key));
+        }
+    }
+
+    #[test]
+    fn keys_are_unique() {
+        let mut seen = std::collections::HashSet::new();
+        for (k, _) in EMBEDDED {
+            assert!(seen.insert(k.as_str()), "duplicate key {:?}", k);
+        }
+    }
+
+    #[test]
+    fn get_returns_embedded_without_override() {
+        clear_overrides();
+        let raw = get(ContentKey::Promotions);
+        assert!(raw.contains("xp_thresholds"), "embedded promotions JSON expected");
+    }
+
+    #[test]
+    fn register_overrides_embedded_then_clear_reverts() {
+        clear_overrides();
+        let custom = r#"{"xp_thresholds":[1,2,3],"heal_on_promote_percent":0.0}"#;
+        register(ContentKey::Promotions, custom);
+        assert_eq!(get(ContentKey::Promotions).as_ref(), custom);
+        clear_overrides();
+        assert!(get(ContentKey::Promotions).contains("xp_thresholds"));
+        assert_ne!(get(ContentKey::Promotions).as_ref(), custom);
+    }
+}
diff --git a/src/simulator/crates/mc-core/src/lib.rs b/src/simulator/crates/mc-core/src/lib.rs
index c1a80f9e..8a821913 100644
--- a/src/simulator/crates/mc-core/src/lib.rs
+++ b/src/simulator/crates/mc-core/src/lib.rs
@@ -7,6 +7,7 @@ pub mod derived_stats;
 pub mod building_state;
 pub mod civic;
 pub mod combat_balance;
+pub mod content;
 pub mod damage_channel;
 pub mod diplomacy;
 pub mod collectibles;