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/vst/reaper_bridge.cpp

@@ -29,10 +29,12 @@ namespace Steinberg {
 // interface (FUnknownPtr uses the iid), so the definition lives with its sole use.
 DEF_CLASS_IID(Steinberg::IReaperHostApplication)
 
-// The "reasampler" ext-state namespace is the SHARED wire contract between the
-// extension (writer) and this instrument (reader); it lives in ext_keys.h (pure,
-// REAPER-free) — reasampler::kProjExtNamespace — so the two artifacts read one symbol
-// and cannot drift. The S1 spike duplicated it locally; that duplication is retired.
+// The ext-state namespace is the SHARED wire contract between the extension (writer)
+// and this instrument (reader); it lives in ext_keys.h (pure, REAPER-free) —
+// reasampler::kProjExtNamespace() — so the two artifacts read one symbol and cannot
+// drift. Channel-derived (Phase V, V4): the accessor returns "reasampler" (stable) or
+// "reasampler_beta" (beta), matching whatever the extension wrote. The S1 spike
+// duplicated it locally; that duplication is retired.
 
 namespace reasampler::vst {
 
@@ -83,7 +85,7 @@ std::optional<std::string> ReaperBridge::readReasamplerExtState(const std::strin
     // complete, else grow and retry up to a 16 MB ceiling.
     for (int cap = 1 << 16; cap <= (1 << 24); cap <<= 2) {
         std::vector<char> buf(static_cast<std::size_t>(cap), '\0');
-        const int rv = getProjExtState_(proj, kProjExtNamespace, key.c_str(),
+        const int rv = getProjExtState_(proj, kProjExtNamespace(), key.c_str(),
                                         buf.data(), cap);
         if (rv <= 0) return std::nullopt;  // absent / empty key
         std::string s(buf.data());