perf(playback): route playhead updates outside pipeline store

[joaner]

Description

This PR separates high-frequency playhead updates from the low-frequency Zustand pipeline store, reducing React re-render pressure during MCAP playback.

Playback time channel

• Add Player.getCurrentTime() and PlaybackControlsApi.getCurrentTime() for imperative, real-time playhead reads.
• IterablePlayer no longer writes activeData.currentTime on every playback tick into useMessagePipelineStore.
• Discrete events (init, seek, pause, loop/end, close) still refresh activeData.currentTime via _emitState().
• Periodic pipeline emits (_maybeEmitPipelineState, 200 ms) skip updates when only time would change; they still propagate slow metadata (isPlaying, speed, progress, etc.).

Call-site updates

• PlaybackBar: initialize playhead ref from player.getCurrentTime().
• AlignPanel: follow playhead via subscribeCurrentTime() instead of useMessagePipeline currentTime.
• Extension context: getSnapshot() still includes currentTime as a compatibility snapshot, built from getCurrentTime() at read time.

Render isolation

• PlaybackOverlayHost: memoize per-overlay items so extension subtree re-renders are bounded.
• SidebarExtensionHost: memoize host + content wrapper.

Documentation

• API.md / API.zh.md: document getCurrentTime(), clarify high- vs low-frequency playback APIs.
• ARCHITECTURE.md / ARCHITECTURE.zh.md: document the dual-channel model (pipeline store vs subscribeCurrentTime / getCurrentTime).

Motivation / related issue

During Studio playback, useMessagePipeline subscribers were waking on every tick because activeData.currentTime changed continuously in the Zustand store. That drove unnecessary React reconciliation (~110 scheduler tasks/s in traces) even for components that only needed slow metadata (bounds, isPlaying, progress).

This change keeps the pipeline store for slowly changing metadata and routes the playhead through subscribeCurrentTime() / getCurrentTime(), matching the pattern already used by 3D/Image panels.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing behavior to change)
• Documentation update
• Refactor / internal cleanup (no behavior change)

Checklist

• npm run lint passes with no errors
• npm test passes (unit tests)
• npm run build and npm run build:lib succeed
• New behavior is covered by tests (or explain why tests aren't applicable)
• Documentation updated (README, API.md, EMBEDDING.md) if the public API changed
• Breaking change: all affected call sites updated and migration path described in PR description

API compatibility

Additive, non-breaking

Symbol	Change
Player.getCurrentTime()	New — returns latest playhead time without subscribing
PlaybackControlsApi.getCurrentTime()	New — same, exposed on extension context
PlaybackControlsApi.getSnapshot()	Unchanged signature — currentTime is now a point-in-time snapshot, not tick-driven
Player.subscribeCurrentTime()	Unchanged — still the preferred high-frequency channel

Migration for embedders / extensions

• Do not drive per-frame UI from getSnapshot().currentTime or useMessagePipeline(s => s.playerState.activeData?.currentTime).
• Do use subscribeCurrentTime() (refs, canvas, throttled listeners) or getCurrentTime() for one-off reads.
• Keep useMessagePipeline for slow metadata: presence, topics, bounds, progress, isPlaying, speed.

No exports removed.

Test plan

• IterablePlayer: getCurrentTime() reflects seek/playhead; pipeline store currentTime does not advance on pure playback ticks
• MinimalPlayer: getCurrentTime() returns zero time while ready
• Manual: open Studio, play MCAP — scrubber/playhead updates smoothly; extensions (marker overlay) remain responsive
• Manual: seek / pause / loop — getSnapshot().currentTime and pipeline store still update on discrete events

Screenshots / recordings

N/A — performance-oriented internal change; no intentional visible UI change. Optional: before/after Performance trace comparing performWorkUntilDeadline rate during playback.