42: Implement first-run onboarding coach marks

[Saber5656]

Derived from docs/issues/42-onboarding-coach.md — the canonical source of truth. Execution order & dependencies: docs/ISSUE_PLAN.md (PR #1). If this issue and the doc disagree, the doc wins.

Summary

Build src/app/onboarding.ts: a one-time, three-step dismissible coach (slider → locate →
menu/share-import) shown on first visit, per DESIGN §3.1.

Context

The time slider is a novel control; a 10-second nudge materially improves first-session success.
Must be skippable, tiny, and never shown again after dismissal.

Scope

• Trigger: after first map idle on / when localStorage["chronomap.onboarded"] !== "1" AND
  no deep-link params present (arriving via share = user already has context; skip and mark done).
• Three sequential coach marks (anchored tooltips with scrim cutout — CSS only, no lib):
  1. slider dock: i18n onboard.slider (スライダーで年代を移動)
  2. LocateButton: onboard.locate (現在地から時間旅行を始める)
  3. MenuButton: onboard.menu (リンク共有や貼り付けはこちら)
     Controls: onboard.next (次へ) / onboard.done (はじめる) / onboard.skip (スキップ,
     step 1 only, text button).
• Any outside interaction (map gesture) = skip-all + mark done.
• Set the flag on completion OR skip; never re-show (Language toggle later does not re-trigger).
• Reduced-motion: no pulse animations; static highlight.

Detailed Requirements

1. Coach must not block the slider/map hit-testing outside the scrim cutout — implement cutout
   via 4 scrim rects around the target (pointer-events on scrim only).
2. Focus management: coach steps are keyboard reachable (Next focused on open); Esc = skip-all.
3. Anchored positioning reuses 33's popover placement helper (flip near edges).
4. Total added JS ≤ 6 KB (lazy import() after idle — §14 budget).

Acceptance Criteria

• e2e: fresh context → 3 steps navigable → flag set → reload shows nothing.
• e2e: deep-link entry (?lat=…) → no coach, flag set.
• e2e: map pan during step 2 → coach gone, flag set.
• Keyboard path (Enter through steps, Esc skip) verified; reduced-motion static rendering.

Validation

tests/e2e/onboarding.spec.ts (fresh storage state per test).

Dependencies

13, 19, 12 (targets exist), 33 (placement helper), 39 (strings).

Non-goals

Feature tours beyond 3 steps, re-showable help center (About covers reference content, 47),
analytics on completion (none).

Design References

DESIGN §3.1, §8 #13, §14.