Reflect M0 landing in docs

Retarget README to ReaSampler and current build targets; reconcile
CLAUDE.md repo-identity + targets table to reaper_reasampler; archive
Milestone 0 from PLAN.md into new COMPLETED.md.

COMPLETED.md

@@ -0,0 +1,32 @@
+# COMPLETED.md — ReaSampler landed milestones
+
+Completed milestone entries removed from `PLAN.md`. Each entry preserves its
+original Goal, Verify, and checklist points with boxes marked done.
+
+---
+
+## Milestone 0 — Transition scaffold: reaper_mpeview → ReaSampler
+**Goal:** Retire the MPE scaffold and stand up the sampler's pure core in its
+place, preserving the pure-core / REAPER-shell split.
+**Verify:** `cmake -B build -S .` configures clean; `cmake --build build` builds
+the renamed extension target and the pure-core test target; `ctest --test-dir
+build` is green with the new `bank_model` + `peaks` suites present.
+
+- [x] Delete `src/mpe_model.{h,cpp}` and `src/mpe_view.{h,cpp}`; remove
+  `tests/test_mpe_model.cpp`.
+- [x] Rename the CMake `project()` and the extension MODULE target from
+  `reaper_mpeview` to `reaper_reasampler` (binary `OUTPUT_NAME` likewise);
+  update `PREFIX ""` / platform SUFFIX blocks to the new target name.
+- [x] Replace the pure `mpe_model` static lib + `mpe_model_tests` executable with
+  `bank_model` (pure static lib) + `bank_model_tests`; keep the CTest wiring.
+- [x] Repoint `src/main.cpp`: drop the `mpe_view.h` include and all `MpeView_*`
+  calls (toggle / IsOpen / OnTimer / Cleanup); stub the extension entry so it
+  loads, logs to console, and registers nothing MPE-specific. The
+  `command_id` / `gaccel` / `hookcommand` registration *pattern* is preserved for
+  reuse (CLAUDE.md §REAPER extension contract) — the MPE action string is removed.
+- [x] Choose and record the persistent action-id prefix for the sampler family
+  (replaces `CEREBELLUM_MPEVIEW_TOGGLE`); this string is forever-stable once
+  shipped (CLAUDE.md §action registration).
+- [x] Refresh `README.md` layout/next-step sections to the sampler module set.
+  (Landed-work reflection is doc-keeper's; this point exists so the stale MPE
+  README does not mislead the first implementer.)