feat(vst): S18 VST3 channel isolation — beta ReaSampler 9000 pairs with beta extension only

Fork the VST3 class UID (beta UID minted+frozen), on-disk name, and display
name by the one channel bit via app_version; couple UID selection to the
channel at compile time so a refactor can't split identity from data.

tests/test_app_version.cpp

@@ -74,6 +74,40 @@ static void testChannelDerivedIdentityStrings() {
     }
 }
 
+static void testVstIdentityStringsForkByChannel() {
+    // S18: the VST3 instrument's on-disk name and display name fork from the SAME one channel
+    // bit as the extension's idents above. Stable values are BYTE-IDENTICAL to pre-S18 — any
+    // drift in the stable branch orphans a saved instance's on-disk reference / mislabels the
+    // FX browser. These are the accessors vst_entry.cpp's factory, the editor title, and the
+    // embed label all source; each branch is the assertion the OTHER config's build would fail.
+    if (isBeta()) {
+        CHECK(vstOutputName() == "reasampler_9000_beta");
+        CHECK(vstPluginName() == "ReaSampler 9000 beta");
+    } else {
+        CHECK(vstOutputName() == "reasampler_9000");
+        CHECK(vstPluginName() == "ReaSampler 9000");
+    }
+    // The on-disk name must match the CMake OUTPUT_NAME fork (REASAMPLER_VST_OUTPUT_NAME): a
+    // divergence between this accessor and the artifact name would ship a binary whose
+    // self-identification disagrees with its filename. (The CMake side is the authoritative
+    // artifact name; this pins the in-binary derivation to the same two literals.)
+    CHECK(vstOutputName() == (isBeta() ? "reasampler_9000_beta" : "reasampler_9000"));
+}
+
+static void testVstIdentityAndDataNamespaceShareOneChannel() {
+    // The S18 pairing invariant at the SEAM: the VST's plugin identity (its display/output
+    // names) and the DATA namespace its bridge reads (extStateNamespace(), what ext_keys
+    // delegates to) must resolve to the SAME channel — a beta-named plugin reading the stable
+    // namespace, or vice versa, is precisely the split the invariant forbids. Both forks fan
+    // out from the one isBeta() bit, so this assertion fails if EITHER fork regressed
+    // independently (a beta output name paired with the stable namespace trips the beta arm).
+    const bool identityIsBeta =
+        (vstPluginName() == "ReaSampler 9000 beta") && (vstOutputName() == "reasampler_9000_beta");
+    const bool dataIsBeta = (extStateNamespace() == "reasampler_beta");
+    CHECK(identityIsBeta == dataIsBeta);          // identity and data agree on the channel
+    CHECK(identityIsBeta == isBeta());            // and both agree with the compiled bit
+}
+
 static void testChannelQualifiedIdAndNameComposition() {
     // The two composition helpers the shells funnel through. A representative shipped id
     // (CAPTURE_TRACK) and phrase must compose to the exact channel-qualified strings — this
@@ -205,6 +239,8 @@ int main() {
     testVersionConstantRendersExactString();
     testChannelDerivedRendering();
     testChannelDerivedIdentityStrings();
+    testVstIdentityStringsForkByChannel();
+    testVstIdentityAndDataNamespaceShareOneChannel();
     testChannelQualifiedIdAndNameComposition();
     testStampClassifiesAsStampedOnOwnChannel();
     testParseWellFormed();