Tracking #1383: S1 version token (A1) - BigInt-safe file versioning #1388

Tracking issue for upstream PR #1383 (branch feat/version-token-s1).

Part of the file-write safety series — plan: easonLiangWorldedtech/Zoo-Code#33, epic: #1375. Item: S1 / A1 — Version token.

Scope

• New src/utils/versionToken.ts:
  • computeVersionToken(filePath): Promise<string> — one fs.stat(path, { bigint: true }); rejects with the underlying ENOENT for absent files.
  • versionTokenOfStat(stats: BigIntStats): string — pure, zero I/O; token format dev:ino:size:mtimeNs:ctimeNs, all five fields exact BigInt values rendered as decimal strings (no float anywhere).
• A1 infrastructure only: no production callers — behavior change is zero. S2 (observation registry) and S4 (guarded write) build on it.

Acceptance

• Same disk state → same token, deterministic across processes; any size/identity/mtime/ctime change → different token
• Regression test pins a size > Number.MAX_SAFE_INTEGER; sub-ms test asserts an exact 1_000 ns delta
• CI green (ubuntu unit-test gate), 100% codecov patch coverage

Status

• PR: OPEN, MERGEABLE — CI green (platform-unit-test ubuntu SUCCESS, codecov/patch SUCCESS)
• CodeRabbit finding (numeric stat precision) fixed via the { bigint: true } route (commit 2c1582bd9), inline comment replied