docs: 1.0 documentation restructure

Split root CLAUDE.md into 19 per-directory files scoped to their source area.
Roll v0 history into docs/ARCHIVE.md; retire CONTEXT.md, CONTEXT-ARCHIVE.md,
PLAN.md, COMPLETED.md. Move plan docs under docs/. Rescue 9 live deferrals
into docs/TODO.md.

src/core/version/CLAUDE.md

@@ -0,0 +1,20 @@
+# src/core/version — version/channel identity
+
+## Scope
+
+REAPER-free version/channel identity consumed by both artifacts (the REAPER
+extension and the VST3 instrument) to derive binary names, ext-state namespaces,
+command-id prefixes, action-name prefixes, dock idents, and version display
+strings.
+
+## Modules
+
+- `app_version` — REAPER-free version/channel identity: CMake-sourced semver constant, ext-state stamp value, and the full set of channel-derived identity accessors. All channel strings derive from one `REASAMPLER_CHANNEL_IS_BETA` bit; no scattered `#ifdef`s in the shells.
+
+## Gotchas
+
+- All channel strings derive from one `REASAMPLER_CHANNEL_IS_BETA` bit — route new
+  channel-specific behavior through this module's accessors rather than adding a
+  scattered `#ifdef` in a shell. See root `CLAUDE.md` "Beta channel build" for the
+  full fan-out (binary name, ext-state namespace, command-id prefix, action-name
+  prefix, dock ident, version display).