Tier rail 2/4: render the standalone tier rail (event-driven, accessible) #157

Part of #131 (remediation steps 2, 3, 4 and 5). Depends on Tier rail 1/4 (the v3 contract).

Replace the standalone canvas glass HUD (screen.js ~2370-2522) with a compact segmented tier rail.

Scope

• Lit segments show currentTier; a distinct notch/outline marks topTier. Do not encode difficulty by changing the component's physical size.
• Readable detail on hover, focus and via an accessible label, e.g. Tier 2 of 4 · full detail at 3. The current canvas is pointer-inert and has no accessible name, so this likely means moving off a bare canvas or adding an accessible layer; decide and state which.
• Make it event-driven: update on song load, phrase transition, mastery change and settings change, not every requestAnimationFrame frame (the current HUD redraws every frame although its inputs change rarely).
• Use FeedBack tokens and the same tier semantics and current/upcoming treatment that Section Map will use.
• Keep per-instance state isolated for Split Screen / player-context instances (see PLAYER_CONTEXT.md).

Acceptance criteria

• A first-time user can identify the current tier and the full-detail threshold without documentation.
• Missing phrase data shows no rail rather than a zero-tier value.
• Keyboard and screen-reader users can inspect current and upcoming values.
• No difficulty-guide work runs continuously at 60 fps (verify with a test or a profiling note).
• The glass HUD can stay available behind the old setting until sub-issue 4 removes it, so this can ship incrementally.

Out of scope

The setting rename (sub-issue 3), Section Map rendering (sectionmap#14), removing glass code (sub-issue 4).