Apply architecture recommendations #7–12

[veillette]

Issue 7 — Remove dead videoUrlProperty from SimModel; the video URL was
managed entirely inside VideoPlayerNode and the model property was never
read or written.

Issue 8 — Avoid full node reconstruction on every playback frame:
• TrackListPanel: skip row rebuild when track IDs are unchanged (i.e. only
points were added, not tracks added/removed).
• DataTableNode: separate structure rebuild (track add/remove) from value
update (time/unit change). Time changes now mutate Text.string in-place
instead of creating new SceneryStack nodes, and VBox children are only
reassigned when the set of visible tracks changes.

Issue 9 — Simplify TrackRowNode binding: remove the manual
if (value !== shouldBeChecked) guard in the model→checkbox link.
Axon Properties deduplicate same-value writes natively, so the guard was
both redundant and an implicit documentation debt.

Issue 10 — Add dispose() to AutoTrackerNode. The timeupdate and
seeked listeners added to the video element were never removed, causing
a leak if the node were ever reconstructed. Fields boundVideoElement
and boundOnFrame are stored so dispose() can call removeEventListener.

Issue 11 — Replace the magic > 0.016 threshold in VideoPlayerNode.step()
with a strict !== equality check. HTMLVideoElement.currentTime is stable
when paused, and Axon deduplication handles same-value writes during
playback, so the arbitrary 16 ms threshold was unnecessary.

Issue 12 — Document the intentional non-reuse of track symbols (A–Z) in
SimModel with a comment explaining the rationale (stable labels for data
export and user recognition).

https://claude.ai/code/session_01VtzmsYvk3TJFGNX9Hk9AEm