refactor(player): E.18 — extract three transport-coordination modules (#581)

Cluster of three small thematically-related cuts in one PR (per the new
'cluster, don't single-shot' convention):

  - `src/store/seekTargetState.ts` — the seek-target guard (`seekTarget`,
    `seekTargetSetAt`, `SEEK_TARGET_GUARD_TIMEOUT_MS`, set/clear/get
    accessors) that suppresses stale Rust progress ticks until the
    engine catches up to the requested position
  - `src/store/togglePlayLock.ts` — the 300 ms double-click cooldown
    behind a `tryAcquireTogglePlayLock()` helper that auto-releases on
    a timer (collapses the three-line inline pattern in `togglePlay`)
  - `src/store/loudnessReseed.ts` — the full `reseedLoudnessForTrackId`
    pipeline (gen-bump → cache + backfill wipe → state reset → server
    row delete → forced seed enqueue), pulled out of playerStore as a
    single async helper

All three were file-private; no caller-side changes outside
playerStore's own imports. The progress handler's seek-guard branch is
now ~3 lines shorter and reads through accessors. `togglePlay` collapses
to one guard check.

24 tests across the three modules pin the API contracts.

playerStore 3189 → 3139 LOC.

src/store/seekTargetState.ts

@@ -0,0 +1,39 @@
+/**
+ * Tracks the target time of the most recent user seek. The progress
+ * handler reads this to suppress stale Rust progress ticks until the
+ * engine has actually caught up to the new position — without the guard,
+ * the slider snaps back briefly because Rust keeps emitting the old
+ * position until the seek finishes propagating.
+ *
+ * Lifetime: set when a seek IPC is issued, cleared once progress reaches
+ * within ~2 s of the target or when the guard timeout (5 s) elapses.
+ */
+
+export const SEEK_TARGET_GUARD_TIMEOUT_MS = 5000;
+
+let seekTarget: number | null = null;
+let seekTargetSetAt = 0;
+
+export function setSeekTarget(seconds: number): void {
+  seekTarget = seconds;
+  seekTargetSetAt = Date.now();
+}
+
+export function clearSeekTarget(): void {
+  seekTarget = null;
+  seekTargetSetAt = 0;
+}
+
+export function getSeekTarget(): number | null {
+  return seekTarget;
+}
+
+export function getSeekTargetSetAt(): number {
+  return seekTargetSetAt;
+}
+
+/** Test-only: reset both mutables so each spec starts fresh. */
+export function _resetSeekTargetStateForTest(): void {
+  seekTarget = null;
+  seekTargetSetAt = 0;
+}