[M1-DET-01] Deterministic mode + sim math rules (G-R8 trait, substreams, sim-source scan)

[offdev]

Deterministic mode + sim math rules (FR-1.4, S-7, PRD §10.3; ARCH-010; M1-DET-01 scope, nothing else).

What this lands

• G-R8 compile-time trait — new public header src/laige-sim/include/laige/sim/determinism.h: SimMathBackend (FixedPoint16_16 default / FloatPinned32), DeterminismConfig, detail::IsDeterminismSafe<T> (integers, enums, fpx16_16, float as the fp32_pinned Scalar, the four SimMath<B>::Vec2/Vec3; double never safe), and LAIGE_DETERMINISM_SAFE(Type, Members...) with a static_assert at the mark site. Enforced by a third static_assert in World::registerSystem over the declared I/O, with an actionable message.
• PRNG substreams — World::Options.seed/deterministic; registerSystem derives Prng::deriveSubstream(seed, systemId) per system (id 0 = master, never assigned); runSystems hands the stream to SystemContext.rng (advanced in place — the stream state IS the replay state).
• Config surface + backend selection — EngineConfig appends seed (u64) + DeterminismConfig (existing 3-member aggregate inits keep compiling); parseEngineConfig gains seed (0..2^53 exact-double bound; 2^53+2 is the smallest rejectable) + determinism (4 new rejection events, unknown nested key → warn + ignore, first failure wins); Engine::create registers the backend-matching built-in first and builds the matching PresentationSnapshot (zero added allocations — headless setup stays exactly 3); run_started gains seed/determinism/math (the laige-run CLI summary line is unchanged; --replay remains the M1-DET-02 stub).
• Sim-source determinism scan — tools/laige-determinism-lint (Python 3 stdlib): D1a/D1b/D1c raw float/double, D2 unordered_*, D3 malformed markers, over src/laige-sim/** with comments/literals/raw strings stripped; same-line // LAIGE-DETERM-EXCEPTION: G-R8 <reason> markers are the documented false-positive policy, every suppressed line counted + printed (EXC-006). New determinism-lint CI job in both workflows + 3 CTest fixture tests in tests/tools.
• Tests — tests/laige-sim/determinism_tests.cpp (14 cases; ctest -R determinism_mode): a trivial moving-entity sim produces bit-identical FNV-1a per-tick state hashes over 256 ticks in two consecutive runs (same build, same seed); different seed diverges; substreams match Prng::deriveSubstream exactly and are independent; deterministic == false → ctx.rng == nullptr; backend selection both ways; the full config-key table. tests/laige-sim/compile_fail/ (4 fixtures; ctest -R trait_compile): 1 positive + 3 negative, each asserting exit code and the G-R8 stderr fragment.
• Docs (same change) — new docs/concepts/determinism.md (the ARCH-010 scope statement) + docs/api/determinism.md; updates to engine.md, entity.md, system_registry.md, sim_math.md, prng.md, testing.md, building.md, module READMEs, index docs. laige-api.json regenerated (573 → 588 symbols). Roadmap checkbox + change-log line + progress board.

Verify (all green)

Command	Result
ctest --test-dir build (Debug GCC 16.2.1)	55/55
ctest --test-dir build-asan (ASan+UBSan)	55/55, leak-free
ctest --test-dir build-tsan (halt_on_error=1)	55/55
ctest -R determinism_mode	1/1 (14 gtest cases)
ctest -R trait_compile	4/4
ctest -R determinism-lint	3/3
python3 tools/laige-include-lint	OK (1/10 deps)
python3 tools/laige-determinism-lint	OK — 17 files, 0 violations, 15 marked exceptions
./build/bin/laige-api-scanner --root . --check laige-api.json	OK — 588 symbols

Zero new warnings under NFR-8.10. CI YAML validated (job present in both workflows).

Notes / limitations

• Deviation (surfaced, not silent): declared dependency M1-CFG-01 has NOT landed — the seed/determinism keys sit on the provisional parseEngineConfig surface (documented in engine.md, the header preamble, and the change-log line); M1-CFG-01 owns the final versioned schema.
• Scope is same-build bit-identity (ARCH-010): cross-target proof lands with M1-DET-04, replay execution with M1-DET-02, stream-state introspection in the state hash with M1-DET-03.
• Windows/macOS/MSVC verification is delegated to the P0 CI jobs (full suite + both lints run there).
• 15 exception markers in-tree (EXC-006), all listed in the CI output on every run: engine.cpp (ADR 0003 JSON number policy), determinism.h (the trait's own float registration), entity.h/system.h/system_timing.cpp (wall-clock diagnostics, ARCH-009), presentation.h (wall-clock alpha).