feat(@projects/@magic-civilization): 🏗️ p3-26 B3 (3/4) — FFI + harness boot the improvement defs

Runtime wiring so improvements actually build + yield in a real headless game:
- GdPlayerApi::set_improvement_defs_json — loads the improvement defs (JSON array of
  {id, build_turns, yields}) onto GameState. Mirrors set_ecology_species_json.
- player_api_main._apply_improvement_defs — stamps DataLoader.get_all_improvements() via the
  FFI at boot (after load_state_json), emitting improvement_defs_api_loaded.

With this, the full B3 chain is live: BuildImprovement → pending (build_turns) → build-tick
completes → city_improvements → process_improvement_yields folds food/production in. gdext
compiles; dylib rebuild in progress.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/game/engine/scenes/headless/player_api_main.gd

@@ -239,6 +239,24 @@ func _hydrate_player_api(num_players: int) -> void:
 	# AFTER `load_state_json`. Without this the ecology phase is a no-op (barren world).
 	_apply_ecology_species()
 
+	# p3-26 B3: stamp improvement definitions so the headless turn can build (build-tick)
+	# and yield (process_improvement_yields) tile improvements. Same `#[serde(skip)]`
+	# re-stamp pattern — load AFTER `load_state_json`. Without this improvements never
+	# build or yield in headless play.
+	_apply_improvement_defs()
+
+
+## p3-26 B3: stamp improvement definitions (DataLoader's improvements: id, build_turns,
+## yields:{food,production}) onto `GdPlayerApi` via `set_improvement_defs_json`. Consumed by
+## the headless improvement build-tick + `process_improvement_yields`.
+func _apply_improvement_defs() -> void:
+	var defs: Array = DataLoader.get_all_improvements()
+	if defs.is_empty():
+		_emit_protocol_error("improvement defs empty — headless improvements will not build/yield")
+		return
+	var n: int = int(_api.set_improvement_defs_json(JSON.stringify(defs)))
+	_emit_event("improvement_defs_api_loaded", {"count": n})
+
 
 ## p3-26 gap 2: stamp the natural-event category configs (DataLoader's merged
 ## `{category: {base_frequency, severity_weights, tiers, …}}`) onto `GdPlayerApi` via

src/simulator/api-gdext/src/player_api.rs

@@ -179,6 +179,18 @@ impl GdPlayerApi {
             .load_ecology_species_json(json.to_string().as_str()) as i64
     }
 
+    /// p3-26 B3: load improvement definitions (a JSON array of improvement
+    /// objects `[{id, build_turns, yields:{food,production}}, …]` from
+    /// `public/resources/improvements/*.json`) so the headless turn can build
+    /// (build-tick) and yield (process_improvement_yields) tile improvements.
+    /// Returns the count loaded, or 0 on parse failure. Call AFTER
+    /// `load_state_json` (the field is `#[serde(skip)]`).
+    #[func]
+    pub fn set_improvement_defs_json(&mut self, json: GString) -> i64 {
+        self.state
+            .load_improvement_defs_json(json.to_string().as_str()) as i64
+    }
+
     /// Stamp the runtime `UnitsCatalog` (id → `UnitStats`) onto the held
     /// `GameState`. Distinct from `set_units_catalog_json` (which loads the
     /// tactical `ai_unit_catalog`): this is the same `mc_units::UnitsCatalog`