Schedule slice 2: atomic cutover in timer.js

[benabraham]

Parent

#10

What to build

Atomic cutover: replace flat position-and-clock fields in timerState with the Schedule module from #11. End-user behavior must remain identical — this is a pure refactor, but a risky one because every read and write site in src/lib/timer.js shifts at once.

Changes:

• Drop phase, currentPeriodIndex, timestampStarted, timestampPaused from initialState. After this slice timerState shrinks to { periods, types }.
• Switch every read of those four fields to the corresponding Schedule computed.
• Switch every write (across all ~28 mutation sites) to the corresponding Schedule verb.
• Wrap mixed verbs that touch both Schedule and Periods (moveToNextPeriod, moveToPreviousPeriod, addPeriod, etc.) in batch() so reactive subscribers don't see torn state.
• Migrate persistence: the storage effect() reads a computed() snapshot of Schedule.snapshot + periods; loadState parses persisted shape into Schedule + periods at boot.
• Update src/lib/timer-simple.test.js to read phase/index via Schedule computeds.

updateTimerState's timerProperties branch becomes dead code after this slice but is not deleted yet — that's slice 3 (#TBD).

Acceptance criteria

• phase, currentPeriodIndex, timestampStarted, timestampPaused are no longer in initialState; timerState is { periods, types }
• All reads of those four fields go through Schedule computeds
• All writes go through Schedule verbs
• Mixed Schedule+Period verbs are wrapped in batch()
• Storage round-trips correctly: a persisted state from before this slice loads into the new shape (or, if backward compatibility is out of scope per project memory, fresh persistence works)
• src/lib/timer-simple.test.js and src/lib/schedule.test.js both pass
• pnpm test and pnpm run build both pass
• Manual QA passes for: start, pause, resume, complete (last period), navigate forward, navigate backward, edit Current Period duration, edit Past Period duration, edit Future Period duration, reset, browser reload mid-run, browser reload while paused
• No UI flicker or torn states during transitions
• No end-user behavior change vs. main

Blocked by

• Blocked by Schedule slice 1: build module + unit tests (not yet wired) #11