De-stubs the headless improvement pipeline:
- improvement_phase::process_improvement_build_phase — ticks pending_improvements down,
completes at 0 → appends the improvement id to the owning city's city_improvements (so it
yields). Registered in END_OF_TURN_PHASES (ecology, healing, improvement_build).
- dispatch::build_improvement — replaces the stub: validates via the action gate, finds the
worker's owning city (CityState.owned_tiles, else first city), queues a PendingImprovement
with turns_remaining from improvement_defs[id].build_turns. (improvement_id, previously
dropped by the dispatcher, is now plumbed through.)
- apply_end_turn repopulates the fresh processor's improvement_yield_table from
state.improvement_defs each turn, so process_improvement_yields actually folds food/
production into cities (was a no-op — table never loaded in real play).
Tests: build-tick (3), dispatch queue (1), registry order. mc-turn 279/0, mc-player-api 136/0.
Remaining (3/4): FFI + harness to boot improvement_defs from public/resources/improvements.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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