[M1-ECS-03] Archetype SoA storage

[offdev]

M1-ECS-03 · Archetype SoA storage

Implements roadmap/M1-heartbeat.md M1-ECS-03 (scope only — no query API,
no iteration, no G-R4 thresholds):

• Archetype = ordered component set, stored SoA: one packed T[]
  column per component, rows kept in ascending slot-id order (a pure
  function of the world state; the convergence property M1-ECS-05 will
  iterate).
• entity→archetype map: two dense per-slot tables (archetypeOf_ 2 B +
  rowOf_ 4 B; no hash) → get<T>(e) / has<T>(e) are O(1).
• addComponent<T>(e, v) create-or-update / removeComponent<T>(e)
  no-op-ok: pool-backed moves (tail memmove + rowOf_ re-sync) with
  zero heap allocation per operation — bounded reserve policy
  (initial min(16, capacity) rows, ×2 growth capped at world capacity,
  every reserve accounted + logged; log2(capacity/16)+1 bounded).
• Budgets kMaxArchetypes = 256 / kMaxArchetypeComponents = 32
  (BudgetExhausted + rate-limited warns ecs/archetype_budget,
  ecs/component_limit); unregistered type → InvalidArgument +
  ecs/component_unregistered; stale handle → the M1-ECS-01 contract.
• Per-slot bookkeeping 5 → 11 B (entity.md updated); destroy/clear
  detach rows first (documented O(tail × row-stride) cost, no allocation).

Tests — archetype CTest entry, 25 cases: basics, stale/unregistered
matrix, layout properties (per-column contiguity, 32 B alignment,
slot-ordered addresses, shift semantics), move data preservation,
two-world layout convergence, budget boundaries (256 sets / 32 components /
growth cap at odd capacity), warn-once rate-limit summary, stats/bytes
tracking, and the 10k-entity churn: 20k add/remove ops in seeded random
order — zero failures, zero reservation delta, zero process-wide
allocations (test-only operator new counter on non-sanitizer trees;
sanitizer trees prove it leak-free), p99/p50 ≈ 1.98 (flat), and a
machine-greppable archetype-churn <stats> line on every ctest run
(CORE-001). Measured baseline (g++ 16.2.1, single-threaded headless):
Debug p50 0.123 ms/op, Release p50 0.0021 ms/op.

Docs: new docs/api/archetype.md (contract + Performance section),
cross-refs in entity.md / component_registry.md / docs index / sim
README; laige-api.json regenerated (443 symbols); roadmap checkbox,
Progress Board 3/25, one-line Change Log.

Verify: canonical g++ tree zero-warning; full ctest 36/36;
ctest -R archetype green; sim suites green on build-asan (ASan+UBSan,
leak-free churn), build-clang, build-release, build-shared,
build-tsan; tools/laige-include-lint OK.

Base commit: 452ab86.