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.

tests/test_view_fx_park.cpp

@@ -4,9 +4,11 @@
 // The properties under test: a mode switch leaves its per-FX offline work here,
 // so a second switch arriving before the first drained must leave every track in
 // the state the SECOND switch specifies — never the first's, never both replayed;
-// a cancel must not strand the pre-park FX state it was the last record of; and
-// the restore/park kind split must hold, restores detaching whole while parks
-// stay queued across the ticks that apply them one FX at a time.
+// a cancel must not strand the pre-park FX state it was the last record of; the
+// restore/park kind split must hold, restores detaching whole while parks stay
+// queued across the ticks that apply them one FX at a time; and the coalescing
+// delay debounces from each enqueue's own time rather than firing once per the
+// first.
 
 #include "../src/shell/view/view_fx_park.h"
 
@@ -35,6 +37,32 @@ static const FxParkIntent* intentFor(const FxParkQueue& q, const std::string& gu
     return nullptr;
 }
 
+// -- the coalescing gate ------------------------------------------------------
+
+static void testCoalesceDebouncesFromEachEnqueuesOwnTimeNotTheFirst() {
+    double readyAt = parkReadyAt(0.0, 1.0);
+    CHECK(!parkIsReady(0.9, readyAt));
+
+    // A second enqueue arrives before the first's delay elapsed. A true debounce
+    // re-arms from THIS call's time; a one-shot would leave readyAt at 1.0 and
+    // this enqueue would have no effect.
+    readyAt = parkReadyAt(0.5, 1.0);
+    CHECK(!parkIsReady(1.0, readyAt));  // a one-shot would already be ready here
+    CHECK(parkIsReady(1.5, readyAt));
+}
+
+static void testRapidAToBToAStillCostsOneWaitFromTheLastFlip() {
+    // Three enqueues inside one delay window (A→B→A) still produce exactly one
+    // wait, measured from the LAST enqueue — not three separate timers and not
+    // one anchored to the first.
+    double readyAt = parkReadyAt(0.0, 1.0);
+    readyAt = parkReadyAt(0.3, 1.0);
+    readyAt = parkReadyAt(0.6, 1.0);
+
+    CHECK(!parkIsReady(1.5, readyAt));
+    CHECK(parkIsReady(1.6, readyAt));
+}
+
 // -- tests -------------------------------------------------------------------
 
 static void testParkEnqueuesOneIntentCarryingNoOps() {
@@ -379,6 +407,8 @@ static void testAReEntrantParkCancelsOnlyWhatIsStillPending() {
 }
 
 int main() {
+    testCoalesceDebouncesFromEachEnqueuesOwnTimeNotTheFirst();
+    testRapidAToBToAStillCostsOneWaitFromTheLastFlip();
     testParkEnqueuesOneIntentCarryingNoOps();
     testParkReportsNothingCancelledWhenNoIntentWasPending();
     testParkOnItsOwnPendingParkReportsNothingCancelled();