feat(@projects): ✨ update building entity port status to partial

Co-Authored-By: Lilith Autocommit <noreply@atlilith.com>

.project/objectives/p2-72a-building-entity-port.md

@@ -2,15 +2,38 @@
 id: p2-72a-building-entity-port
 title: "Port NPC Building entity (lairs/villages/ruins) into Rust"
 priority: p2
-status: open
+status: partial
 scope: game1
 category: architecture
 owner: simulator-infra
 created: 2026-05-11
-updated_at: 2026-05-11
+updated_at: 2026-05-12
 blocks: [p2-72a-save-format-migration, p2-72a, p2-72]
 ---
 
+## STATUS — 2026-05-12
+
+**Stage 2b (Rust surface) landed.** `BuildingEntity` lives in
+`mc_core::building`, `GameState.npc_buildings: Vec<BuildingEntity>` is
+the canonical mirror, and `GdGameState` exposes the full accessor surface
+(count / dict / all / spawn / remove / set-visited / convert-type /
+clear). Round-trip serde test green; `cargo check -p
+magic-civ-physics-gdext` clean.
+
+**Stage 2b acceptance dropped to `partial`, not `done`.** The
+GDScript→Rust spawn-path wiring (per § E) is deferred to Stage 4
+(`p2-72a-gdgamestate-canonical-render-source`). Reason: no singleton
+`GdGameState` autoload exists today — every current GDScript caller
+instantiates a fresh `GdGameState` per op, so dual-writing spawns into
+those ephemeral instances would not flow through to any persistent
+mirror. The Rust spawn API is in place and ready for Stage 4 to wire,
+which is the legitimate place for the singleton to land.
+
+`Building.gd` is annotated with a Stage-4 marker comment but otherwise
+unchanged. Read-paths (renderer, AI, encyclopedia, fauna, climate-event
+handlers) continue to consult `GameState.npc_buildings` (GDScript array)
+unchanged.
+
 ## STATUS — 2026-05-11
 
 Filed as prerequisite for `p2-72a-save-format-migration` after Wall 3 of
@@ -165,21 +188,78 @@ fn building_entity_round_trip_is_byte_identical() {
 
 ## Acceptance
 
-- ☐ `BuildingEntity` Rust struct lives in `mc-core::building` (or
-  `mc-buildings` if Wave 1 picks crate split).
-- ☐ Serialize/Deserialize round-trip unit test passes.
-- ☐ `api-gdext` exposes the accessor surface in § C.
-- ☐ Every Game 1 spawn path routes through `GdGameState::spawn_npc_building`.
-- ☐ Every Game 1 read path routes through `GdGameState::npc_building_at` / `npc_buildings_all` / `set_npc_building_visited` / `remove_npc_building`.
-- ☐ `Building.gd` deleted (option b) or thinned to a view-only wrapper (option c) — Wave 1 picks.
-- ☐ `GameState.npc_buildings` and `GameState._npc_buildings_by_tile` deleted from `game_state.gd`. Spatial index lives in Rust only.
-- ☐ Hex renderer / unit movement / AI tactical / city borders / encyclopedia / siege flow all continue to work; visual + GUT verification.
-- ☐ `cargo check --workspace` green.
-- ☐ `cargo test --workspace` green (modulo known pre-existing
-  `solo_dominion` errors).
-- ☐ GUT headless run green.
+- ✓ `BuildingEntity` Rust struct lives in `mc-core::building` (chose
+  option ii — extend `mc-core` rather than new crate; see Stage 2b
+  decision log below).
+- ✓ Serialize/Deserialize round-trip unit test passes
+  (`cargo test -p mc-core building::` — 3 tests green:
+  `entity_round_trip_is_byte_identical`,
+  `entity_position_serializes_as_pair`,
+  `entity_round_trip_city_placement_defaults`).
+- ✓ `api-gdext` exposes the accessor surface in § C
+  (`npc_building_count`, `npc_building_dict(idx)`, `npc_buildings_all`,
+  `spawn_npc_building`, `remove_npc_building_by_id`,
+  `set_npc_building_visited`, `convert_npc_building_type`,
+  `clear_npc_buildings` — see
+  `src/simulator/api-gdext/src/lib.rs` Stage 2b block).
+- ☐ Every Game 1 spawn path routes through `GdGameState::spawn_npc_building`
+  — **deferred to Stage 4** (no singleton; dual-writes into
+  per-call `GdGameState` instances would be ephemeral).
+- ☐ Every Game 1 read path routes through `GdGameState::npc_building_at`
+  / `npc_buildings_all` / `set_npc_building_visited` /
+  `remove_npc_building` — **Stage 3+ (save-format migration) /
+  Stage 4 (canonical-render-source) scope, not Stage 2b.**
+- ☐ `Building.gd` deleted (option b) or thinned to a view-only wrapper
+  (option c) — **Stage 4 scope**. Stage 2b adds a marker comment
+  flagging the upcoming Stage 4 conversion.
+- ☐ `GameState.npc_buildings` and `GameState._npc_buildings_by_tile`
+  deleted from `game_state.gd`. Spatial index lives in Rust only.
+  **Stage 4 scope.**
+- ☐ Hex renderer / unit movement / AI tactical / city borders /
+  encyclopedia / siege flow all continue to work; visual + GUT
+  verification — Stage 2b doesn't touch read paths, so existing GDScript
+  flow is unaltered; Stage 4 acceptance.
+- ✓ `cargo check --workspace` green (only pre-existing
+  `unsafe_op_in_unsafe_fn` / `missing-docs` warnings).
+- ✓ `cargo test -p mc-core -p mc-turn` green
+  (mc-core: 243/243 + 3 golden; mc-turn: see Stage 2b verification log).
+  Pre-existing failures: `mc-ai` test crates (`gpu_walltime`,
+  `gpu_tree_integration`, `mcts_basic`, `clan_rollout_divergence`,
+  `ultimate_lookahead_stress`, `budget_enforcement` — all
+  `AbstractPlayerState._pad_fr/_pad_rel` missing-field errors); these
+  reproduce on the unmodified pre-Stage-2b tree (verified via
+  `git stash && cargo check --workspace --tests`).
+- ☐ GUT headless run green — Stage 2b changes only Rust + a doc
+  comment in `building.gd`; GUT unchanged. Re-run at Stage 4 close.
 - ☐ `p2-72a-save-format-migration` `blocked_by:` updated to remove this
-  objective once it lands.
+  objective once it lands — **partial unblock**: Stage 3 can begin
+  consuming the `BuildingEntity` type and `GameState.npc_buildings`
+  field, but full unblock awaits Stage 4 wiring.
+
+## Stage 2b decision log — 2026-05-12
+
+**Crate decision: option (ii) — extend `mc-core`.** Rationale:
+`mc_core::lair` already lives in the same crate and owns adjacent
+per-tile siege state; `BuildingEntity` is pure data + queries with no
+turn-tick lifecycle of its own; no new crate-level surface to justify.
+Spec § A explicitly prefers (ii).
+
+**Position serde shape: tuple `(i32, i32)` round-tripped as a JSON
+array `[col, row]`** via a small `axial_pos_as_pair` adapter. Matches
+`Building.gd::to_dict`'s `"position": [position.x, position.y]` exactly
+so Stage 3 save-format migration gets byte-identity with existing
+GDScript-emitted saves.
+
+**Placement serde shape: lowercase string `"map"` / `"city"`**
+(`#[serde(rename_all = "lowercase")]`). Matches `Building.gd::from_dict`
+shape.
+
+**`name` kept on the entity (not derived from data).** Spec § B
+suggested dropping it, but two production sites
+(`ecological_event_handlers_b.gd::_abandon_lair`, `fauna.gd::_abandon_lair`)
+mutate `b.name = "Ruin"` alongside `b.type_id = "ruin"` mid-game, so the
+field is mutable state, not a pure data-lookup. `convert_npc_building_type`
+takes both `type_id` and `name` for parity.
 
 ## Out of scope
 

src/game/engine/src/entities/building.gd

@@ -3,6 +3,17 @@ extends RefCounted
 ## Game entity for both city buildings and map-placed NPC buildings (lairs, villages, ruins).
 ## City buildings: placement="city", owner=player_index, position=city tile.
 ## NPC buildings: placement="map", owner=-1, position=map tile.
+##
+## p2-72a Stage 2b: The Rust source-of-truth for this entity now lives in
+## `mc_core::building::BuildingEntity` and the canonical `npc_buildings`
+## mirror sits on `mc_turn::GameState.npc_buildings`. This GDScript class
+## stays unchanged for Stage 2b — no singleton `GdGameState` bridge exists
+## yet, so dual-writing into per-call `GdGameState` instances would be
+## ephemeral. Stage 4 (`p2-72a-gdgamestate-canonical-render-source`)
+## introduces the singleton, at which point this script collapses to a
+## thin view over `GdGameState::npc_building_dict(idx)` and the spawn
+## paths in `village_lair_placer.gd` / `ecological_event_handlers_b.gd`
+## start routing through `spawn_npc_building` / `convert_npc_building_type`.
 
 ## Unique instance ID (e.g. "lair_42", "building_granary_3")
 var id: String = ""