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/version_generated.h.in

@@ -1,9 +1,23 @@
 #pragma once
 // version_generated.h.in — configure_file TEMPLATE. CMake substitutes @REASAMPLER_VERSION@
-// (the ONE source-of-truth variable in CMakeLists.txt) and writes the result to the build
-// tree as version_generated.h. DO NOT edit the generated header — edit REASAMPLER_VERSION
-// in CMakeLists.txt; that single edit re-generates this and re-threads the exact string
-// (leading zero preserved verbatim) into the binary constant, the ext-state stamp, and the
-// show-version action. See app_version.h / .cpp.
+// (the ONE version source-of-truth variable in CMakeLists.txt) and @REASAMPLER_CHANNEL_IS_BETA@
+// (0 for the stable/default build, 1 for -DREASAMPLER_CHANNEL=beta) and writes the result to
+// the build tree as version_generated.h. DO NOT edit the generated header — edit
+// REASAMPLER_VERSION / the channel flag in CMakeLists.txt; that single edit re-generates this
+// and re-threads the exact string (leading zero preserved verbatim) plus the channel bit into
+// the binary constants, the ext-state namespace/stamp, the command-id prefix, and the panel
+// readout. See app_version.h / .cpp.
+//
+// Two channel-derived facts flow from here and NOWHERE ELSE, so "what makes a beta a beta" is
+// one auditable definition (Phase V, V4 — beta-in-isolation):
+//   * REASAMPLER_VERSION_STRING — the numeric semver triple, IDENTICAL on both channels
+//     (leading zero preserved). This is the ext-state STAMP value: it stays the numeric triple
+//     so parseVersion classifies it as Stamped when its own channel reads it back, and it is
+//     byte-identical to stable's stamp regardless of channel (the channel is carried by the
+//     ISOLATED namespace, not baked into the stamp — see app_version.h).
+//   * REASAMPLER_CHANNEL_IS_BETA — 0 (stable, the default absent-flag build) or 1 (beta). The
+//     one bit app_version fans out into every channel-qualified identity (display suffix,
+//     namespace, command-id prefix, action-name prefix, binary/dock idents).
 
 #define REASAMPLER_VERSION_STRING "@REASAMPLER_VERSION@"
+#define REASAMPLER_CHANNEL_IS_BETA @REASAMPLER_CHANNEL_IS_BETA@