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_paint.cpp

@@ -51,11 +51,9 @@ void ReaSamplerEditor::paint(HDC hdc, const RECT& dirty) {
 
     // A meter frame's dirty rect lies wholly inside the bar field, and nothing else is drawn
     // there — so the field is all that needs redrawing, and the rest of the face is still in the
-    // back buffer from the last full compose. Decided GEOMETRICALLY rather than by a flag,
-    // because Windows coalesces a meter invalidate with any other pending one into a single
-    // union rect: a flag would then paint a meter frame over a face that had really changed.
-    const Rect& mf = meterRects_.field;
-    if (!mf.empty() && dx >= mf.x && dy >= mf.y && dr <= mf.right() && db <= mf.bottom()) {
+    // back buffer from the last full compose. Decided GEOMETRICALLY rather than by a flag — see
+    // meterFastPathEligible (master_meter) for why.
+    if (meterFastPathEligible(meterRects_, Rect::ltrb(dx, dy, dr, db))) {
         paintMeterField(bmp);
     } else {
         // Repopulated by paintDeck when the meter is on screen; left empty by the empty state.
@@ -83,7 +81,8 @@ void ReaSamplerEditor::paint(HDC hdc, const RECT& dirty) {
         }
 
         // A sheet layered over the face covers the meter, so the fast path must not paint
-        // through it — dropping the rect routes the next meter tick down this path instead.
+        // through it even if something else invalidates a rect that happens to match the
+        // stale field bounds — invalidateMeter itself already skips while covered (see there).
         if (view_ == View::kBrowse || curvePopup_ != CurveTarget::kNone) meterRects_ = {};
     }