view: assert the FX-park coalescing delay's debounce; correct four overclaiming doc/comment claims

Extracts parkReadyAt/parkIsReady as a tested pure fold per PLAN.md's phase
criterion; the rest is wording fixes — hitch bound, hazard width, forced-drain
scope, progressive CPU reclaim.

src/shell/view/view_fx_park.cpp

@@ -33,14 +33,19 @@ namespace {
 // swallow a rapid A→B→A flip whole — that flip's restore then cancels the park
 // outright and the plugins never move at all — and short enough that the window
 // in which a user could add an FX to the track the switch just hid, and so miss
-// the pre-park snapshot, stays implausible. Seconds, not minutes: this delay is
-// the width of the hazard documented at tickDeferredFxParks.
+// the pre-park snapshot, stays implausible. Seconds, not minutes; this is ONE
+// term of the hazard's width — see tickDeferredFxParks for the other.
 constexpr double kParkCoalesceSeconds = 1.0;
 
 FxParkQueue g_queue;
 ReaProject* g_owner = nullptr;  // the project the pending intents were enqueued against
 bool        g_draining = false;
-double      g_parkReadyAt = 0.0;  // time_precise() stamp; re-armed by every enqueued park
+// The gate is GLOBAL, not per-track: a park enqueued for track Z re-arms this
+// same stamp even though it does nothing to an already-partial park on track A.
+// Benign — that track's FX are already I_FXEN=0/B_MAINSEND=0 from the
+// synchronous half, so only its CPU reclaim (not correctness) is delayed — but
+// this is not a per-intent debounce.
+double g_parkReadyAt = 0.0;  // parkReadyAt() stamp; re-armed by every enqueued park
 
 // Makes "one drain at a time" explicit rather than implied by the call sites.
 // RAII because an apply can throw and a stuck flag would silence the queue for
@@ -81,10 +86,11 @@ int firstOnlineFx(MediaTrack* tr) {
     return -1;
 }
 
-// One track by GUID. The park tick resolves ONE track and does so afresh every
-// tick — a whole-project map would be rebuilt per tick for a single lookup, and
-// caching the handle across ticks is exactly the dangle ValidatePtr2 is here to
-// prevent.
+// One track by GUID, resolved afresh every tick — caching the handle across
+// ticks is exactly the dangle ValidatePtr2 is here to prevent. Still O(tracks),
+// still one guidString() per track per tick; a per-tick unordered_map keyed by
+// GUID would cost the same enumeration to build for a single lookup, so the only
+// thing avoided by NOT building one is that allocation.
 MediaTrack* findTrack(ReaProject* proj, const std::string& guid) {
     const int count = CountTracks(proj);
     for (int i = 0; i < count; ++i) {
@@ -169,7 +175,7 @@ std::vector<FxOfflineOp> deferFxPark(ReaProject* proj, const std::string& guid)
     adoptOwner(proj);
     // Re-armed, not set once: while switches keep arriving the park keeps
     // waiting, which is the whole point of coalescing them.
-    g_parkReadyAt = time_precise() + kParkCoalesceSeconds;
+    g_parkReadyAt = parkReadyAt(time_precise(), kParkCoalesceSeconds);
     return g_queue.park(guid);
 }
 
@@ -239,7 +245,7 @@ void drainDeferredFxRestores() {
 
 void tickDeferredFxParks() {
     if (!drainGateOpen()) return;
-    if (time_precise() < g_parkReadyAt) return;  // still coalescing
+    if (!parkIsReady(time_precise(), g_parkReadyAt)) return;  // still coalescing
 
     const std::string guid = g_queue.nextParkGuid();
     if (guid.empty()) return;  // restores only — not this half's work