P2b: offline WAV render of the arrangement

docs/P2.md

@@ -18,9 +18,15 @@ Verification:
 
 Limits: exactly four sections of two bars; no insert/reorder/length; select is not undoable; no per-section mute.
 
-## P2b · render (next)
+## P2b · render
 
-Offline render of the 8-bar arrangement to a WAV inside the project, with a command and a surface control.
+`zhila_engine::render_arrangement(RenderRequest)` drives the same `AudioState` offline: no cpal device, the same voices, sample faces, gains, master and the 8-bar sections, plus a 1.5 s tail. `Command::render` is intercepted by the host, rendered in `spawn_blocking`, written with `hound` as 48 kHz stereo 32-bit float to `<project>/renders/<project>-<ms>.wav`, and announced with `Event::rendered { path, duration_sec }`; the command result is the unchanged state. Render is not a project edit and does not push history.
+
+Surface: `render` in the top rail; the wire logs the send and the `rendered` path; the button's title keeps the last path.
+
+Verification: `cargo test -p zhila-engine` (non-silent 16.5 s buffer, correct frame count); `node scripts/render-smoke.mjs` (status 16.5 s, peak 0.442, 6.3 MB, float 48 kHz stereo, event duration matches); `npm --prefix ui run render-ui-smoke` (button → wire path, screenshot `preview-p2-render.png`).
+
+Limits: WAV only, stereo float, fixed 48 kHz and 1.5 s tail; no stems, normalisation or section ranges.
 
 ## P2c · agent (next)
 

docs/PLAN.md

@@ -1,7 +1,7 @@
 # ZHILA — план проекта
 
 Черновик v1.1 · 1 ноября 2026
-Статус: **P2 в работе** (P0, P1a–P1f, P2a; см. `P0.md`, `P1.md`, `P2.md`). Демо `reference/demo/` — референс, не кодовая база. Осталось: рендер WAV и агент.
+Статус: **P2 в работе** (P0, P1a–P1f, P2a, P2b; см. `P0.md`, `P1.md`, `P2.md`). Демо `reference/demo/` — референс, не кодовая база. Осталось: агент.
 
 **Зафиксировано 1 ноября:** descent — основной вид; камень = паттерн; климат — на жилу (концепт погоды остаётся); платформа v1 — Linux; UI — Svelte 5. Журнал — §9.
 

docs/PROTOCOL.md

@@ -172,3 +172,7 @@ The Tauri app starts the same host in-process, so the wire contract is unchanged
 ## P2a additions
 
 `State` adds `arrangement: Pattern[]` (four sections in this phase) and `active_section: u8`; `State.pattern` is the active section. Commands `select_section { index }`, `clear_section { index }`, `copy_section { from, to }`; event `arrangement { sections, active }`. Pattern step/face commands edit the active section. The project row stores `arrangement` and `active_section`; old projects with only `pattern` are migrated to four copies of it.
+
+## P2b additions
+
+`Command::render` (no arguments). The host renders the current arrangement to a WAV in the project, answers with the unchanged state, and broadcasts `rendered { path, duration_sec }`. The render is not journaled and does not push undo. The file is 48 kHz stereo 32-bit float.