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/app/CLAUDE.md

@@ -0,0 +1,35 @@
+# src/app — REAPER extension entry point
+
+## Scope
+
+Contains only `main.cpp`. Since the Phase Q hoists (Q-W3 onward), this TU is ONLY
+pointers + entry + dispatch — the actual capture/panel/persist/action orchestration
+lives in `shell/`. `main.cpp` owns: receiving REAPER's dispatch struct
+(`ReaperPluginEntry`), resolving the REAPER API function pointers
+(`REAPERAPI_LoadAPI`), the globals other files reference via `extern` (`g_hInst`,
+`g_rec`), the `ReaSamplerSession` instance, its own bindable-action family via the
+Q-W6 data-driven registration table (`shell/actions/action_registry`), and invoking
+the other action families' (`design_view` / `bank` / `ingest`) own
+register/handle/unregister triples at load and unload.
+
+Exactly **one** translation unit defines `REAPERAPI_IMPLEMENT` — that is `main.cpp`.
+Every other `.cpp` includes `reaper_plugin_functions.h` without the define and gets
+`extern` declarations for the global API function pointers.
+
+See root `CLAUDE.md`'s "REAPER extension contract" section for the full four-step
+action-registration contract (`command_id` / `gaccel` / `hookcommand` / unload
+mirror-unregister) that both this file's own action-table rows and the other
+families' register/handle/unregister triples follow.
+
+## Modules
+
+- `main.cpp` — the REAPER extension's entry point and the sole `REAPERAPI_IMPLEMENT` TU; see root `CLAUDE.md`'s "REAPER extension contract" section for the registration contract this file implements.
+
+## Gotchas
+
+- This is intentionally a thin TU post-Phase-Q. Adding a new bindable action to
+  `main.cpp`'s own family means adding one row to its `ActionTableRow` table and a
+  flat handler function — do not hand-roll a parallel register/hookcommand/unregister
+  mechanism alongside the table.
+- Never let a second `.cpp` define `REAPERAPI_IMPLEMENT` — that would double-allocate
+  the global REAPER API function pointers.