[M1-CFG-01] Declarative game config

[offdev]

Closes roadmap/M1-heartbeat.md step M1-CFG-01 (FR-1.5, PRD §7.1/§10.2/§10.3, ARCH-007).

What this PR does

The declarative game config surface — the version 1 config.json schema, its loaders, the programmatic override merge, and the debug-build-only hot reload of non-simulation keys.

• New public header src/laige-sim/include/laige/sim/config.h (+ config.cpp):
  • EngineConfig moved from engine.h — the five original members keep their order (existing aggregate initializers compile unchanged), gaining the budgets block (system_time_default_ms, draw_calls_per_frame, particles_per_frame — declared values before their M2 consumers), the camera block (fov/zoom/follow-lerp — stored, consumed in M2), and asset_roots (non-empty strings; no existence check — the M2 asset pipeline owns it).
  • The REQUIRED version key is checked before any other key (missing → config/version_missing, non-integer → config/version_invalid, ≠ 1 → config/version_unsupported); the provisional M1-HEAD-01 documents migrate by adding "version": 1.
  • Unknown keys at any level warn config/unknown_key and are ignored (forward-compat); first failure wins in document order; every rejection is a rate-limited warn with the NFR-13.3 5-field text + InvalidArgument.
  • loadGameConfig(path) — bounded 1 MiB read + the M0-CORE-07 parse + schema validation; laige-run now uses it (exit codes unchanged).
  • EngineConfigOverride + applyConfigOverride — the FR-1.5 override-of-a-subset merge (per-leaf optionals, each set field validated, first set field that fails wins, value semantics).
  • ConfigHotReloader — move-only, no thread, caller-driven poll; debug builds only (release rejects with config/hot_reload_disabled): non-sim changes apply in place + config/hot_reload_applied; sim-affecting changes are refused atomically + config/hot_reload_rejected (old/new fields, config untouched).
• Replay identity preserved: the configHash covers only the sim-affecting fields, so the encoding (tag 1) is unchanged and every committed baseline / replay log stays valid (replay.h/.cpp comments + docs updated).
• Docs: new docs/api/config.md (key table, versioning + migration, rejection table, override API, hot-reload contract, Performance, misuse warnings); engine.md / replay.md / determinism.md / testing.md / docs-README / sim-README cross-refs updated to the final surface.
• Tests: new game_config CTest entry (38 tests: every rejection domain, the version gate, first-failure-wins, the file loader, the override merge, the hot-reload contract incl. the release-disabled path); engine_tests drops the migrated EngineConfigParse.*; fixtures gain "version": 1 (headless_smoke, samples/hello, the replay smoke fixtures). laige-api.json regenerated (734 symbols).
• Roadmap bookkeeping: M1-CFG-01 box checked; board 20/25 (38 total); change log row + retroactive M1-DET-05 row (the M1-ECS-06 precedent).

Verification

• ctest -R config green (config_json + game_config + hello_config_valid)
• Full ctest 88/88 on build (Debug g++), build-asan (leak-free), build-release (the hot_reload_disabled path exercised), build-clang, build-tsan (config suites, halt_on_error=1), build-shared
• tools/laige-include-lint OK, tools/laige-determinism-lint OK (24 sim sources, 0 violations), laige-api-scanner --check clean
• Zero new warnings under NFR-8.10

Untested paths

• The MSVC _fsopen read branch (CI-only; the M1-DET-02 precedent).

Compatibility

No existing symbol or behavior changed: the provisional keys carry over unchanged under the version 1 schema, the EngineConfig member order is preserved, the replay configHash encoding is unchanged, and the CLI exit codes are unchanged.