Move the bounded undo/redo stacks, snapshot factory, scroll-top reader
registry, and pending-scroll-top channel into a dedicated module under
src/store/. playerStore re-exports the three public APIs that QueuePanel
and the test harness already imported (registerQueueListScrollTopReader,
consumePendingQueueListScrollTop, _resetQueueUndoStacksForTest), so no
caller-side changes are needed.
The new module adds explicit push/pop accessors so the undo/redo store
actions stop reaching into module-scoped arrays directly. Inline
`pendingQueueListScrollTop = …` writes inside applyQueueHistorySnapshot
become `setPendingQueueListScrollTop(…)` calls — same effect, scoped
through the module.
PlayerState gets the `export` keyword so queueUndo.ts can type its
state-shaped parameters.
playerStore.ts 3618 → 3576 LOC. queueUndo gains a focused test file
covering snapshot deep-cloning, the redo-stack invalidation on a fresh
undo push, max-size enforcement, and the scroll-top channel.
Pre-PR check: PASS.
Frank Stellmacher