Timeline plugin: playback controls

[sandesh-sp]

Replaces the playback placeholders with the real controls: skip to start/end, step back/forward, play/pause, and a button cycling the speed multiplier through 1x, 2x, 4x, 0.5x. Play and speed are hidden when a mission turns playback off.

Stack — merge in order, starting from the base:

1. Timeline plugin: shell and component placeholders #239 shell and component placeholders → development
2. Timeline plugin: timeline view #240 timeline view
3. Timeline plugin: date selector #241 date selector
4. Timeline plugin: playback controls #242 playback controls
5. Timeline plugin: time mode control #243 time mode control

[sandesh-sp]

Review pass over the stack (#239–#243). Pushed as 2221522, rebased on #239–#241.

Worth flagging up front: the playback engine these controls drive lives in TimelineAdapter.tsx, which is only ever touched by #239 — so the three engine-side issues were fixed there and this PR carries the UI half.

In this PR

• Icon-only buttons had no accessible name. Five transport buttons carried title alone. Each now has an aria-label, play/pause exposes state through aria-pressed, and the group is labelled.
• Dead affordances at the range endpoints. handleStepForward/handleStepBackward dropped the step when it would overshoot rather than clamping, and the buttons were never disabled — so with currentTime within one step of an endpoint the button looked fully live and did nothing. The adapter now clamps (fixed in Timeline plugin: shell and component placeholders #239), and canStepForward/canStepBackward drive disabled here, so a button only reads as inert once the boundary is actually reached.
• Speed cycle restarts from a value outside the list instead of returning undefined, so the interval can never be handed a zero or negative multiplier.

Fixed in #239 (engine side)

• Play at the end of the range armed an interval that self-cancelled one tick later — the Pause icon showed for a full second at 1x with zero movement. Now rewinds to the start.
• The setCurrentTime updater was impure, calling mmgisEmit('time:changeRequested') and setIsPlaying(false) from inside itself. The bus is synchronous mitt, so the emit ran timeInputChange → layer reload → re-entry into the same hook, from inside a state updater. Both now run in the interval body against refs.
• That change also removed startTime/endTime from the interval's deps, so a core echo mid-playback no longer tears down and rebuilds the interval every tick.

On throttling: I looked at whether 4x playback hammers core too hard and concluded it doesn't — core's own TimeUI ships a 100ms interval on the identical path (TimeUI.js:1433), so this is strictly more conservative than the UI it sits alongside.

tsc --noEmit clean, 898 unit tests pass.