test: cover the tui lifecycle and theme switching

[clappingmonkey]

What & why

Adds test/tui.test.ts, which tests the tui function itself: timers, lifecycle and theme switching. Until now only the pure helpers had tests. The tests call the real tui export with a fake host API and node:test mock timers (Date, setInterval, setTimeout). No source changes.

16 tests covering the rules the plugin relies on:

• startup at night and during the day; toast: false
• a manual theme change survives until the next day/night boundary
• theme not ready yet: retried every 250 ms, with only one retry pending at a time, including while interval ticks keep firing
• theme.set() fails: retried on the next interval, the period isn't marked as handled
• theme not installed: warns once per period
• solar mode switches at sunrise; solar mode without coordinates warns and falls back to fixed hours
• signal already aborted before startup; abort, dispose, and abort followed by dispose all clear the timers
• errors thrown by the host API are caught, and the next tick retries

Closes #

Behavior impact

• Theme-switching logic (periodFor, boundaries, gentle override)
• Plugin options (dayTheme, nightTheme, dayStartHour, nightStartHour,
  checkIntervalMs, toast) — README options table updated
• Lifecycle / timer / toast behavior
• Packaging (package.json exports, files, published contents)
• No user-facing behavior change

Testing

• npm run typecheck passes (TypeScript 7.0.2)
• npm test passes (38 tests)
• Manually verified in opencode (if behavior changed): not needed, tests only

Also:

• passes with TZ set to UTC, Pacific/Kiritimati, America/St_Johns and Australia/Lord_Howe
• mutation check: removing any one guard in tui makes at least one test fail. The guards are the abort listener, onDispose, the aborted-at-start check, the boundary-only check, not-ready/failed not marking the period handled, the ready retry and its single-pending guard, and the try/catch.
• npm pack --dry-run is unchanged (tests aren't published)

Checklist

• PR title follows Conventional Commits
  (enforced by the PR Title check; drives the release-please version bump)
• Defaults still work with zero config
• Docs updated if behavior/config changed (no behavior change)
• No secrets or credentials introduced