P1e: persist undo and redo history across reloads

docs/P1.md

@@ -176,3 +176,27 @@ The waveform is now an editor. A core's sixteen faces are seventeen contiguous b
 - Seventeen contiguous cuts only: no zoom, fade, loop points or per-face gain.
 - The trim is the outer boundaries of the sixteen faces; the original asset is not rewritten.
 - Cut history survives the session but not yet a reload (the next slice).
+
+---
+
+# P1e · history across reloads
+
+The undo stack is part of the project now.
+
+## What is stored
+
+`ProjectEdit` is serialized: bpm, gains, the whole pattern, and every core's seventeen face boundaries. The session keeps up to 64 edits in `history` and a `redo` stack. The store writes both as JSON on the project row (`history`, `redo` columns, migrated with `ALTER TABLE`) every time the session saves; on load the stacks are parsed and handed to `Session::from_project`. `can_undo`/`can_redo` in the state reflect the restored stacks, so the rail buttons light up after a restart.
+
+The snapshot carries the cores' cuts, not the audio; the audio is still content-addressed and decoded from the asset. Undo after a reload therefore restores pattern, tempo, gains and every face range exactly.
+
+## Verification
+
+- `cargo test --workspace`: the core's project-data test and the store test cover the history round-trip; the store test saves an edit with the core boundaries.
+- `node scripts/persistence-smoke.mjs`: after the daemon restart, `can_undo` is true; `undo` reverts the manual cut from 1.23 s to 1.21 s; `redo` restores 1.23 s; the face edge and the pattern/step/tempo are checked.
+- `npm --prefix ui run restored-smoke`: the loaded project shows its core and face; the undo button reflects the restored stack.
+
+## Limits
+
+- Imports are not undoable; the history is snapshots, not the complete journal.
+- The cap is 64 edits; older history is dropped.
+- Corrupt history JSON is ignored with a warning and an empty stack.