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.

src/shell/instrument/editor_platform.cpp

@@ -77,8 +77,20 @@ void ReaSamplerEditor::invalidate() {
 
 void ReaSamplerEditor::invalidateMeter() {
     if (!childHwnd_) return;
+    // The meter is covered (Browse, the curve popup) or has nothing to draw (empty state) —
+    // ballistics still advance in onMeterTimer, but nothing on screen changed, so invalidating
+    // anything here would only buy a whole-client repaint of chrome/waveform/deck the meter
+    // never touches. Distinct from "bounds not resolved yet" below: this is a fact about what
+    // the face is showing, not about whether meterRects_ happens to be populated.
+    const bool meterOnScreen =
+        view_ == View::kSample && curvePopup_ == CurveTarget::kNone && !selectedId_.empty();
+    if (!meterOnScreen) return;
+
     const Rect& f = meterRects_.field;
-    if (f.empty()) {  // nothing cached (see meterRects_) — the whole face has to answer
+    if (f.empty()) {
+        // On screen, but no full paint has resolved its bounds yet (first paint, or a resize
+        // just dropped the cache) — the whole-client fallback is cheap here because the window
+        // is already fully invalid from the resize/creation that caused this.
         invalidate();
         return;
     }
@@ -155,6 +167,10 @@ void ReaSamplerEditor::removedFromParent() {
         childHwnd_ = nullptr;
     }
     releaseBackBuffer();  // a client-area bitmap outlives nothing here
+    // Unreachable today (WM_PAINT outranks WM_TIMER, so a reopen's first paint resolves this
+    // before any meter tick can read it stale) — cleared anyway so the invariant is structural,
+    // not a timing accident, the same reason onSize clears it on resize.
+    meterRects_ = {};
 }
 
 tresult PLUGIN_API ReaSamplerEditor::onSize(ViewRect* newSize) {