P2a: eight-bar arrangement with four editable sections

docs/P2.md

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+# P2 · arrangement, render, agent
+
+P2 turns the loop into a thing that can be played, rendered and performed with an agent. It is delivered in three slices.
+
+## P2a · the arrangement
+
+Four 2-bar sections (A–D) form an 8-bar loop. Each section is a full `Pattern` (5 rows × 16 steps, with per-row faces). The engine walks 64 steps, picks the section and the pattern step, and triggers the section's rows; the bar clock runs 1–8. The state carries `arrangement: [Pattern; 4]`, `active_section`, and `pattern` = the active section for the editors.
+
+Commands: `select_section { index }`, `clear_section { index }`, `copy_section { from, to }`, event `arrangement { sections, active }`. Pattern edits apply to the active section. Select is persisted but not undoable; clear/copy push history. Sections, the active index and their history are stored in SQLite (`arrangement`, `active_section` columns, migrated with `ensure_column`) and restored on load.
+
+Surface: the quarry gets a section strip (A–D with note counts, copy →, clear); the pattern grid edits the active section. The descent draws the 64-step road, bar labels 01–08, section seams and letters A–D, with the active section emphasized; the playhead follows the 8-bar clock.
+
+Verification:
+
+- `cargo test -p zhila-core arrangement`: sections independent, select/copy/clear, undo, material carries four sections, loop_bars = 8.
+- `node scripts/persistence-smoke.mjs`: imports, edits section B, copies A→C, cuts, restarts, verifies the arrangement, active B, B's step 0 = 0, C = A, and undo/redo.
+- `npm --prefix ui run arrangement-smoke`: four sections, select B, edit a step, switch to the nave, play, screenshot `preview-p2-arrangement.png`.
+
+Limits: exactly four sections of two bars; no insert/reorder/length; select is not undoable; no per-section mute.
+
+## P2b · render (next)
+
+Offline render of the 8-bar arrangement to a WAV inside the project, with a command and a surface control.
+
+## P2c · agent (next)
+
+An in-app LLM agent on the same verbs, with state/sound context and approval for project edits.