docs: HANDOFF round-2 review followup + move review 2 into notes/

Append a 'Review followup — Round 2' section to HANDOFF.md documenting
the round-2 Gemini-review fixes:

- Critical: request-reorder desync (SnapshotSequencer assumed client send
  order == server processing order; not true on the wire). Fixed by
  promoting the per-hour pendingHours Set to a single global pendingHour
  mutation lock — serializing toggle POSTs eliminates the reorder window
  entirely.
- High: toggle endpoint guessed user intent from server state, which
  combined with any desync to invert clicks. Fixed by requiring an
  explicit action: 'on' | 'off' field on every request.
- Low: round-2 R2-3 (retry storm re-probes succeeded accounts) noted as
  a deliberate trade-off, not fixed.

Move the round-2 review report from the project root into
notes/claude-reset-review-2.md to match the convention established for
the round-1 report.

Net delta vs branch base: 431 tests (was 427 after round 1; +4 contract
tests for the explicit-action endpoint). Biome and svelte-check clean.

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