feat(durability): atomic writes + registry lock (Theme N1)

[NagyVikt]

Summary

Theme N1 from docs/future/17-ROADMAP.md — eliminate the two known data-loss windows in registry.json / snapshot writes.

• src/infra/fs/atomic-write.ts — temp+rename with fsync(fd) and POSIX fsync(dir) after rename; mode applied before rename so 0600 files are never visible at a looser mode.
• src/infra/fs/registry-lock.ts — O_EXCL advisory lock with PID-liveness probe + 30 s wall-clock stale-lock heuristic.
• persistRegistryAtomic is the single registry write path; it reloads under the lock and unions account entries before writing.
• secureWriteFile now delegates to atomicWriteFile so every snapshot, current, and sessions.json write fsyncs.
• AccountService.useAccount no longer calls saveRegistry directly — public method signatures unchanged.

Exit criteria covered (Theme N1)

• src/infra/fs/atomic-write.ts lands with fsync-before-rename.
• Every fsp.writeFile for registry.json, current, sessions.json, and snapshot files routes through atomicWriteFile (via secureWriteFile).
• src/infra/fs/registry-lock.ts lands with stale-lock reaping (PID liveness + 30 s wall-clock).
• AccountService.persistRegistry is the only write path; it reloads-under-lock then merges deltas.
• src/tests/registry-durability.test.ts simulates the writer dying between writeFile and rename and asserts the registry still parses.
• Release notes in releases/v0.1.25.md include a Durability section.

Verification

npm run build           # tsc clean
npm test                # 88/88 tests pass
node -e "..."           # saveRegistry still exported

Notes

• Sibling PRs from N3 (errors taxonomy) and N4 (paths deprecation) are running in parallel; expect a rebase if they land first.
• Dir-fsync is gated on process.platform !== "win32" since Windows does not allow opening a directory as a file.
• In-process callers that share process.pid block each other instead of reaping the lock; cross-process stale ownership is handled by the liveness probe + 30 s heuristic.

Test plan

• npm run build
• npm test — all 88 tests green, including 3 new ones in src/tests/registry-durability.test.ts
• node -e "require('./dist/lib/accounts/registry.js').saveRegistry" resolves
• Singleton at src/lib/accounts/index.ts still constructs

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