Sandbox mode: keyboard-only entry + synthetic practice events for onboarding tour

[tyler-dane]

Summary

• Programmatic keyboard-only mode entry/exit for the tour's moveFocus/editSequence/targetEvent/nudge steps, reusing the existing keyboardOnlyActions.enter()/exit() store API (already a plain, non-gesture-coupled API — no store changes needed). A single useEffect cleanup guarantees exit on every path (skip, finish, fork exit, step change, Escape, unmount).
• Ephemeral, non-persisted practice events for those same steps, merged into the Week/Day view-model pipeline right before grid consumption — never written to IndexedDB or routed through the mutation pipeline. Read-only enforcement reuses isGridEventContentReadOnly via a new isSandboxReadOnly flag (mirrors the existing isDemo/demoEventIds pattern), gated off for every sandbox event except the one built for the "nudge" lesson.
• useDayEventViewModel gained an opt-in includeSandboxEvents flag so sandbox events only surface from the day grid itself, not from the sidebar's Up Next card (which also consumes that hook for an independent "today" range) — caught in review.

Known follow-up (documented in code + internal brief log)

The nudge lesson's mutation attempt safely no-ops (no error, no toast, nothing persisted) rather than visibly repositioning the practice event, since sandbox events are intentionally never written into the real query cache the mutation pipeline reads. Wiring a visible local reposition would mean touching the shared grid-focus/mutation code this brief was explicitly split out to keep isolated — left as a fast-follow rather than a rushed patch under this PR.

Test plan

• bun run type-check clean
• biome check clean on touched files
• bun run knip clean
• New unit tests: exit-path audit (useOnboardingSandboxKeyboardOnly.test.ts) and sandbox event builder/read-only gating (onboarding.sandbox-events.test.ts)
• Full targeted suite (OnboardingTour, event-query hooks, Up Next, Day view) green — 166 pass
• Independent review pass (code-reviewer agent) — found 2 real issues, both fixed (Up Next leak scoped out; nudge no-op documented as a safe, known limitation)
• e2e for a full lesson with mouse disabled — not added in this PR, flagged for follow-up

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