fix: make controlled month mode actually navigate

[dogmar]

Summary

Audit fix B (2 of 5), follows #12. In controlled month mode the MonthView's Prev/Next buttons did nothing and onMonthChange never fired — there was no way to drive a controlled month. With a stateful parent it was worse: an infinite render loop.

Root cause

The old code only emitted onMonthChange from an effect watching viewingYearMonth, but viewingYearMonth derives from the month prop — which only the parent can change — so navigation could never originate inside the component. And because currentMonth is rebuilt as a fresh object every render, the viewingYearMonth useMemo produced a new reference each commit, firing the effect on every render → setMonth → re-render → loop.

Fix

• Buttons notify directly. goNextMonth/goPrevMonth (controlled) call onMonthChange(adjacentMonth) and then move focus.
• Value-keyed skip ref. The focus-sync effect would otherwise also fire for the button's follow-on focus move (double-notify). A skip keyed on the focus value (not a one-shot boolean) suppresses exactly that move and can't strand — a stale flag would otherwise swallow a later keyboard crossing if the parent ignored the click (caught in pre-commit review).
• Keyboard crossing into a non-visible month notifies from the focus-sync effect (single source for that path).
• The viewingYearMonth effect skips controlled mode, which is what actually breaks the loop.

Tests (7 new, in a controlled month describe)

Single-month next/prev (fire once, view stays until parent updates), multi-month button advance (notification must come from the button since the adjacent month is already visible), the stateful round-trip (would hang if the loop regressed), keyboard crossing in/out of the visible window, and the no-op-click-then-keyboard regression for the stranded-skip hazard.

Verification

• vp run ready exits 0: lint+typecheck 0/0, package 672 tests, website 54.
• Pre-commit review confirmed the ref logic is sound under React 18 batching and StrictMode double-invocation, and found the stranded-skip hazard, which this version fixes (value-keyed) with a dedicated regression test.

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