fix: S12 R2 — marker-drag sets selectedZone_, ADSR rate-resolve, gateAdsr doc

Marker drag now sets selectedZone_ so single-capture controls stay reachable. Lifted/default ADSR rescales nominal 44100-Hz frame counts by sampleRate/44100 via adsrNeedsRateResolve; v4-authored zones skip rescale. gateAdsr documented vestigial.

src/vst/sampler_core.h

@@ -340,11 +340,13 @@ class Voice {
 public:
     // Starts this voice on `note` at `velocity`, playing `sample` (a stable reference
     // the caller must keep alive for the voice's lifetime — the Keymap owns it), repitched
-    // from `rootNote`. `gateAdsr` is the effective Gate AHDSR (the engine supplies the
-    // instrument-wide attack/decay/sustain/release timing; the per-zone HOLD stage comes from
-    // sample.play.adsr.holdFrames, folded in here). The S15 play MODE + Trigger params and the
-    // S16 pitch ENGINE + pitch envelope are read from `sample.play`. The Preserve shifters MUST
-    // already be pre-sized (presizePreserveShifters, off-thread) — start() only reset()s + warm()s
+    // from `rootNote`. All five AHDSR fields (A/H/D/S/R) are read directly from
+    // sample.play.adsr — the per-zone values stamped by buildTier0Keymap / buildZonedKeymap.
+    // `gateAdsr` is the VoiceEngine's instrument-wide ADSR parameter, accepted for interface
+    // compatibility but NOT used by start() (vestigial since the S12 review fix moved A/D/S/R
+    // onto the per-zone SampleData). The S15 play MODE + Trigger params and the S16 pitch
+    // ENGINE + pitch envelope are read from `sample.play`. The Preserve shifters MUST already
+    // be pre-sized (presizePreserveShifters, off-thread) — start() only reset()s + warm()s
     // them (RT-safe, no allocation) since it runs on the audio thread inside process(). The warm
     // silence pass settles the OLA taps before the first output frame (no cold-start click).
     // Byte-identical to the pre-S15 engine when sample.play is default (Gate + Varispeed + no