fix(vst): channel-derive the shared ext-state namespace (V4↔S4 reconcile)

ext_keys.h's kProjExtNamespace now delegates to app_version::extStateNamespace() so the beta instrument reads "reasampler_beta" — the namespace the beta extension writes — instead of stale stable. Link app_version into reasampler_vst.

src/ext_keys.h

@@ -5,18 +5,28 @@
 // wire contract cannot drift between the two artifacts (the S4 reviewer flagged the
 // spike's duplicated constants as a drift risk).
 //
-// PURE HEADER: NO REAPER types, NO VST3 types, NO SWELL, NO vendor/ includes. Just
-// string constants, so both the REAPER-facing persist shell and the SDK-facing VST
-// bridge can include it without pulling either SDK.
+// PURE HEADER: NO REAPER types, NO VST3 types, NO SWELL, NO vendor/ includes. The key
+// spellings are string constants; the NAMESPACE is channel-derived (Phase V, V4) so it
+// delegates to the pure app_version module (also REAPER-free / VST3-free). Both the
+// REAPER-facing persist shell and the SDK-facing VST bridge include this without pulling
+// either SDK.
 //
 // FOREVER-STABLE once shipped: these strings key every already-saved project's
 // stored state. Changing any of them orphans that state. See persist.h for the
 // per-key retirement / migration semantics — this header only owns the spellings.
 
+#include "app_version.h"
+
 namespace reasampler {
 
-// The ext-state namespace all ReaSampler project state is stored under.
-inline constexpr const char* kProjExtNamespace = "reasampler";
+// The ext-state namespace all ReaSampler project state is stored under. CHANNEL-DERIVED
+// (Phase V, V4): delegates to the ONE app_version symbol so the extension (writer) and the
+// VST3 instrument (reader) resolve the SAME namespace per channel — "reasampler" on stable,
+// "reasampler_beta" on the isolated beta build. An accessor (not a constexpr literal)
+// because the value is fixed by the channel bit at build time. This is the wire-contract
+// reconciliation between S4 (shared ext_keys) and V4 (channel-isolated namespace): without
+// it a beta instrument would read the stable namespace and see empty state.
+inline const char* kProjExtNamespace() { return extStateNamespace().c_str(); }
 
 // The multi-bank key: the whole serialized BankBook (pool + named banks). This is
 // the key the VST3 instrument reads to see the live bank (read-only, S4). persist.h