docs(plan): tick S6 checkboxes; mark IReaperUIEmbedInterface must-verify as verified-by-S6

PLAN.md

@@ -249,9 +249,9 @@ as the main editor is reused.
 build:** the `IReaperUIEmbedInterface` contract + embed message/lifecycle against
 `vendor/reaper-sdk/sdk/reaper_plugin_fx_embed.h`.
 
-- [ ] Implement `IReaperUIEmbedInterface` on the VST3; draw a compact keymap/level
+- [x] Implement `IReaperUIEmbedInterface` on the VST3; draw a compact keymap/level
   strip inline in the TCP/MCP using the same LICE surface as the editor.
-- [ ] Embed lifecycle (open/close/resize/hit-test inline) handled cleanly; reflects
+- [x] Embed lifecycle (open/close/resize/hit-test inline) handled cleanly; reflects
   the live keymap/levels.
 
 ## S7 — stereo channel mode (mono | stereo; core channel dimension + bus negotiation)
@@ -495,8 +495,11 @@ artifact, not extension-only.
   path is `IReaperHostApplication::getReaperApi` (resolve-by-name) and
   `getReaperParent(3)` (host context) via `reaper_vst3_interfaces.h`; the
   `0xdeadf00d`/`0xdeadf00e` opcodes are VST2-only and do not apply.
-- **`IReaperUIEmbedInterface`** — embed contract + message/lifecycle, against
-  `reaper_plugin_fx_embed.h` (needed only at S6).
+- **`IReaperUIEmbedInterface`** — ~~embed contract + message/lifecycle, against
+  `reaper_plugin_fx_embed.h` (needed only at S6).~~ **verified (S6):** interface
+  exposed via `queryInterface` on the controller; inline strip drawn into REAPER's
+  embed bitmap (LICE idiom, no HWND); `WM_GETMINMAXINFO` size hints confirmed;
+  embed open/close/resize lifecycle clean.
 - **VST3 bus arrangement (S7)** — `setBusArrangements` / `getBusArrangement` and REAPER's
   mono/stereo instrument-bus expectations, against the vendored Steinberg SDK +
   `reaper_vst3_interfaces.h`. The channel-mode toggle depends on the output bus