[M1-LOOP-02] Presentation snapshot + interpolation state

[offdev]

Implements M1-LOOP-02 — Presentation snapshot + interpolation state (roadmap/M1-heartbeat.md): per tick, the world produces a presentation snapshot of Position2D state; render frames interpolate between the last two tick states with a clamped alpha.

What's here (the step's Scope, nothing else)

• Position2D<Backend> — the first built-in component: the entity's 2D simulation-space position as the selected SimMath backend's Vec2 (ADR 0002). Both backends registered: Position2DFpx16 (fpx16_16, default) / Position2DFp32 (fp32_pinned, opt-in).
• PresentationSnapshot<Backend> (new header-only public header, class template — one instantiation per backend, the M1-ECS-02 pattern):
  • onTick(tick): per completed tick, prev ← curr, curr ← world value for every live entity with a Position2D — over a pre-reserved per-slot table (24 B/slot, one setup-path allocation; no per-tick/per-frame heap, PERF-003).
  • New entities snap to curr (the documented scope behavior): entities added between ticks render at their spawn position, no phantom interpolation; generation-checked so slot recycling self-heals.
  • onRenderFrame(renderNs): the alpha alpha = (R − A(T)) × rate / 10⁹ in exact integer arithmetic (overflow-free for any 64-bit clock reading; no float accumulator, ARCH-010), clamped to [0, 1] — never extrapolates (before the anchor → 0; a clock jump a full tick or more past → 1).
  • sample_position(e) (the roadmap's exact name): lerp(prev, curr, alpha) for synced entities, snap for entities first seen since the last refresh, InvalidArgument + warn-once for stale handles (the World::check precedent), InvalidArgument no-warn for live handles without a Position2D, stopped (no world access, no log) when moved-from.
  • create(world, startReferenceNs, options): validates the tick rate (20–120 Hz, the loop's range) with one rate-limited presentation/tick_rate_invalid warn.
• GameLoop gains the M1-HEAD-01 wiring seam: Options::onTick (plain noexcept function pointer — fires once per completed tick, never for a failed tick) + onTickContext + startReferenceNs() (the alpha's anchor base).
• Docs (same change, CORE-006/DOC-007): new docs/api/presentation.md (full contract + Performance section), docs/api/game_loop.md (hook section, Options table, startReferenceNs), docs/README.md, the module README.
• API manifest: laige-api.json regenerated (555 symbols; api-real-tree green).

Verify (step's Verify command: ctest -R presentation)

• ctest -R presentation — green (10 cases: tick-rate validation; exact Q16.16 linear interpolation at alpha 0/0.5/0.75/near-1 (raw-unit expectations, no float round-trips); the alpha clamp matrix incl. a 285-year clock reading; entity-added-between-ticks snap-then-interpolate; catch-up per-tick refresh; stale/missing rejection; the GameLoop hook integration incl. the failed-tick case; moved-snapshot stop; the 500-entity × 100-frame zero-alloc window (allocs=0, machine-greppable); the fp32 backend).
• Canonical g++ tree: zero-warning, full ctest 45/45.
• build-asan, build-tsan, build-clang, build-release, build-shared: all zero-warning, 45/45 each (the sanitizer trees prove the zero-alloc window leak-free).
• tools/laige-include-lint: OK.
• Roadmap: box checked, Progress Board 12/25 (total 32/193), change log row — all in this PR.