Closes the cross-file checkpoint-atomicity gap in ADR-0156. The four
checkpoint files now live in a committed gen-NNNN/ directory; the single
atomic os.replace of a 'current' pointer file is the commit boundary.
A kill before the pointer swap leaves the prior committed generation
intact; a kill after commits the new generation. Unreferenced gen dirs
are ignored. ADR-0156's deferred parent-dir fsync is also closed.
Key changes:
- engine_state/__init__.py: begin_generation() + commit_generation() +
_resolve_dir() for all load_* methods. Flat-layout legacy checkpoints
migrate into gen-0000 on first begin_generation call. GC retains K=2
committed generations.
- chat/runtime.py: checkpoint_engine_state uses the two-phase commit;
finalize_turn_trace_hash no longer writes discovery_candidates outside
the generation sequence (the second unguarded write path is closed).
- evals/l10_continuity/runner.py: _inject_orphan_tmp updated to inject
the two orphan shapes of the generation model.
- tests/test_adr_0219_generation_checkpoint.py: 18 tests, one per
acceptance-gate bullet + biting mutation variant each.
L10 lane: all_gates_pass=true; versor_condition<1e-6 throughout.
Smoke: 95 passed. Runtime: 20 passed.
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