feat: three-way merge for diff.ts and warmUpMs for StandbyController …

[mctituschristian]

…(#702, #703)

Issue #703 — diff.ts three-way merge conflict detection:

• Add MergeConflictError to src/errors.ts with field, baseValue, localValue, remoteValue properties
• Add mergeInvoices(base, local, remote) to src/diff.ts implementing the three-way merge algorithm:
  • Both branches unchanged → keep base value
  • Only one branch changed → fast-forward to that value
  • Both changed to same value → no conflict, use that value
  • Both changed to different values → throw MergeConflictError
• 17 unit tests in test/diff.test.ts covering all three merge outcomes

Issue #702 — standby.ts configurable warm-up period:

• Add StandbyController class to src/standby.ts with:
  • warmUpMs?: number option (default: 0)
  • inactivityMs?: number option (default: 30_000)
  • During warmUpMs window, inactivity does not trigger standby
  • After warm-up, normal inactivity detection resumes
  • recordActivity() resets the inactivity timer (post warm-up only)
  • standby getter, onStandby(listener), start(), stop()
• 12 unit tests in test/standby.test.ts
  Closes standby.ts: Add configurable warm-up period before standby mode activates #702
  Closes diff.ts: Write tests for three-way merge conflict detection #703

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