feat(version): V4 beta-in-isolation channel via -DREASAMPLER_CHANNEL

Compile-time channel flag forks a fully isolated reaper_reasampler_beta
(namespace, command-id prefix, action names, dock ident, -beta render) from
one auditable app_version definition. Stable identity byte-unchanged.

src/persist.h

@@ -28,9 +28,16 @@
 
 namespace reasampler {
 
-// The ext-state namespace the index JSON is stored under. FOREVER-STABLE once
-// shipped: changing it orphans every already-saved project's index.
-inline constexpr const char* kProjExtNamespace = "reasampler";
+// The ext-state namespace every ReaSampler key is stored under. CHANNEL-DERIVED (Phase V,
+// V4): the pure app_version module owns the one channel-qualified string — "reasampler" on
+// stable (byte-identical to the pre-V4 build) or "reasampler_beta" on the isolated beta
+// build. FOREVER-STABLE per channel once shipped: changing either orphans every already-
+// saved project's state. Beta reads/writes ONLY its own namespace — a project saved by
+// stable shows empty/default state in beta and vice versa; that isolation is the accepted
+// V4 safety property (no cross-namespace read, migration, or fallback), not a bug.
+// Returns const char* (not a constexpr literal) because the string is channel-derived at
+// build time; the accessor is the single call point for all persist reads/writes below.
+inline const char* projExtNamespace() { return extStateNamespace().c_str(); }
 
 // The RETIRED legacy ext-state key: pre-multi-bank projects stored the whole
 // serialized BankIndex here (single bank). Phase B2 no longer WRITES it — on save