Ω-W1-T3 review fixes: skip meter invalidate when covered; push fast-path predicates into core

Fixes the critical finding — Browse/curve-popup no longer trigger 60 Hz
whole-client repaints when the meter is invisible. Pushes the fast-path and
self-containment geometry into master_meter with tests; batches the four minor
findings.

tests/test_master_meter.cpp

@@ -5,6 +5,8 @@
 //    the mono bar taking the whole field, the two stereo bars, all inside the column.
 //  * bar count — the SAME LaneSplit resolveLaneSplit folds, over channel mode x source
 //    channel count, so it can never become a second rule.
+//  * the editor's dirty-rect fast path — meterFastPathEligible's wholly-inside test, and
+//    meterBarsWithinField, the self-containment invariant the fast path rests on.
 //  * the dB axis — top/floor on the field's edges, an interior value, and the clamps.
 //  * ballistics — instantaneous rise, 20 dB/s fall, the 1.5 s hold and its release AT RATE;
 //    the audio thread's clip latch surviving a UI frame; the per-field single-lane fold.
@@ -287,6 +289,29 @@ static void testDrawEqualityCoversTheDrawnQuantities() {
     CHECK(meterDrawEqual(a, graze));
 }
 
+// The editor's WM_PAINT fast-path test: a dirty rect wholly inside the field takes the
+// meter-only redraw; anything that pokes outside it, or a field that hasn't been laid out yet,
+// falls back to the full compose.
+static void testFastPathEligibleOnlyForADirtyRectInsideTheField() {
+    const MeterRects m = meterRects(kColumn, LaneSplit::Single);
+    CHECK(meterFastPathEligible(m, m.field));                    // the whole field
+    CHECK(meterFastPathEligible(
+        m, Rect::ltrb(m.field.x + 2, m.field.y + 2, m.field.right() - 2, m.field.bottom() - 2)));
+    CHECK(!meterFastPathEligible(m, Rect::ltrb(m.field.x - 1, m.field.y, m.field.right(),
+                                               m.field.bottom())));  // pokes left of the field
+    CHECK(!meterFastPathEligible(
+        m, Rect::ltrb(kColumn.x, kColumn.y, kColumn.right(), kColumn.bottom())));  // whole column
+    CHECK(!meterFastPathEligible(MeterRects{}, m.field));  // no cached layout at all
+}
+
+// The self-containment invariant drawMeterField's redraw-the-field-alone shortcut rests on:
+// every bar meterRects() hands back stays inside the field, in both lane splits.
+static void testFieldBoundsEveryBarItPaints() {
+    CHECK(meterBarsWithinField(meterRects(kColumn, LaneSplit::Single)));
+    CHECK(meterBarsWithinField(meterRects(kColumn, LaneSplit::Stereo)));
+    CHECK(meterBarsWithinField(MeterRects{}));  // degenerate: nothing to bound, nothing drawn
+}
+
 static void testDegenerateColumnYieldsNothing() {
     const MeterRects m = meterRects(Rect::ltrb(0, 0, 0, 0), LaneSplit::Stereo);
     CHECK(m.field.empty() && m.barA.empty() && m.barB.empty());
@@ -300,6 +325,8 @@ int main() {
     testEveryOtherTickCarriesANumeral();
     testTheFloorNumeralStaysInsideTheGutter();
     testAColumnTooNarrowForTheInteriorDrawsNothing();
+    testFastPathEligibleOnlyForADirtyRectInsideTheField();
+    testFieldBoundsEveryBarItPaints();
     testPeakRisesAtOnceAndFallsAtTwentyDbPerSecond();
     testPeakHoldSitsForItsFullWindowThenReleases();
     testClipLatchesFromThePublishedFlagAndClearsOnDemand();