Add M3: APPDATA persistence + presets + per-pane distro/label

Backend:
- save_workspace / load_workspace Tauri commands writing to
  %APPDATA%\com.megaproxy.tiletopia\workspace.json with atomic
  tmp+rename. Path from app.path().app_config_dir() (no dirs crate).

Layout helpers:
- tree.ts: changeDistro (with id swap to force XtermPane remount via
  {#key}), changeLabel, presetSingle / TwoColumns / ThreeColumns /
  TwoRows / TwoByTwo.
- New ops.ts with PaneOps interface bundling split / close /
  setDistro / setLabel / distros, drilled through Pane chain
  instead of individual callbacks.

UI:
- LeafPane: in-toolbar editable label (click to rename, Enter
  saves, Esc cancels) and distro chip popover. Picking a different
  distro respawns the pane.
- App.svelte: migrated from localStorage to APPDATA via the new
  Tauri commands, debounced 500ms. One-time localStorage migration
  on boot. Split inherits parent's distro+cwd. Titlebar preset
  buttons with confirm when replacing >1 pane.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

memory.md

@@ -12,15 +12,20 @@ Durable memory for this project. Read at session start, update before session en
 - **Don't commit `node_modules`, `src-tauri/target`, or `.pnpm-store`. DO commit `Cargo.lock`** (binary project, reproducible builds).
 - **Session awareness without an in-pane agent.** Plan: poll `/proc/<pid>/cwd` of the shell's child + foreground process every ~2s. Sufficient to detect `cd` and whether `claude` is running.
 - **State propagation in the layout tree: hybrid mutable + replace.** The root tree is `$state(...)` at App level. Direct mutation (e.g. `node.ratio = X` during gutter drag) is reactive via Svelte 5's deep proxy. Structural changes (split/close) go through pure helpers in `tree.ts` that return a new root, which App reassigns. Drag stays fast (no tree walk); structural changes stay simple. `{#key leaf.id}` around `LeafPane` ensures swapping a leaf in/out cleanly unmounts XtermPane (which kills the PTY on destroy).
-- **Layout persistence: localStorage at App level, key `tiletopia.tree.v1`.** Saves on every `$effect` tick (deep reactivity catches all mutations). Migrating to `%APPDATA%/tiletopia/` is M3.
+- **Layout persistence: `%APPDATA%/com.megaproxy.tiletopia/workspace.json`** via two Tauri commands (`save_workspace`, `load_workspace`). Atomic write (tmp + rename) so a crash mid-save can't leave a partial file. Path comes from Tauri's `app.path().app_config_dir()` — no separate `dirs` crate needed. M2's localStorage path is checked once at boot as a one-time migration source, then cleared.
+- **Auto-save is debounced 500ms.** Every tree mutation kicks the `$effect`; it resets a timeout and only writes after 500ms of quiet. Cheap enough to never need throttling on UI mutations; matters because each gutter-drag step would otherwise hit disk dozens of times per second.
+- **Pane operations bundled into a `PaneOps` interface** in `lib/layout/ops.ts`. Pane and SplitNode just pass `ops` through; LeafPane consumes it. Replaces M2's per-callback prop drilling (would have been split + close + setDistro + setLabel + distros = 5 separate props). Easier to grow as M4 adds broadcast / palette ops.
+- **Per-pane distro change forces a remount via id swap.** `changeDistro` in `tree.ts` assigns a new id to the leaf; `Pane.svelte`'s `{#key leaf.id}` unmounts XtermPane (which kills the old PTY) and mounts a fresh one with the new distro. Same mechanism we already use for split/close.
+- **Split inherits parent's distro AND cwd** (not label — label is a per-pane name, not a hierarchy thing). So "split right" while in a project keeps both panes in that project.
 
 ## Open questions / TODOs
 
 - [x] ~~**M2 — splits-tree layout component.** Two panes side by side, draggable divider, both panes alive. Save/restore layout as JSON.~~ Done 2026-05-22.
-- [ ] **HMR distro picker reset.** Less acute now that distro selection is per-leaf (auto-applied at split time from app-level default). The titlebar `default:` buttons let the user re-set the default at any time. Revisit when adding per-pane distro switching in M3.
-- [ ] **Auto-save debouncing.** Currently every tree mutation writes to localStorage. Dragging a gutter fires many writes per second. Cheap (localStorage is fast, JSON is small) but worth debouncing in M3 when persistence moves to disk.
-- [ ] **M3 — workspace persistence + preset layouts.** Migrate from localStorage to `%APPDATA%/tiletopia/workspaces.json` (via Tauri's `plugin-store` or direct FS). Add preset layouts (3 columns, 2×2 grid). Multi-workspace tabs. Per-pane distro override and pane labels.
-- [ ] **M4 — orchestration.** Broadcast input groups, idle/finish notifications, Ctrl+K fuzzy palette.
+- [x] ~~**M3 — workspace persistence + preset layouts + per-pane distro + pane labels.**~~ Done 2026-05-22.
+- [x] ~~**Auto-save debouncing.**~~ 500ms timer in `App.svelte` `$effect`.
+- [x] ~~**HMR distro picker reset.**~~ No longer an issue — per-pane distro selection via in-toolbar popover; titlebar `default:` only seeds new splits.
+- [ ] **Multi-workspace tabs.** Several independent layouts the user can switch between. Saved as `workspaces.json` with `{ current: id, list: [{ id, name, tree }] }`. Not on the M0–M5 critical path; either bolt on after M5 ship or fold into a "tabs" minor milestone.
+- [ ] **M4 — orchestration.** Broadcast input groups (write same bytes to N PTYs flagged into a group), idle/finish notifications (detect when a `claude` pane stops emitting output for >Ns → toast), Ctrl+K fuzzy palette over `label / distro / cwd`. Will extend `PaneOps` with a couple more methods + add a "broadcast group" concept to the leaf type.
 - [ ] **M5 — Ship.** Replace placeholder icons, NSIS installer, Forgejo release. Copy `claude-usage-widget`'s release scripts.
 - [ ] **Native Windows shells (cmd / pwsh)?** `portable-pty` supports them for free; keep the option open. Decide whether to expose in UI at M3.
 - [ ] **Persistent scrollback across app restarts.** Would need an out-of-process mux daemon. Big scope creep; explicitly deferred past v1.
@@ -28,6 +33,17 @@ Durable memory for this project. Read at session start, update before session en
 
 ## Session log
 
+### 2026-05-22 — M3 persistence + presets + per-pane distro/label
+
+- Backend: added `save_workspace(json)` and `load_workspace()` Tauri commands. Atomic write via tmp + rename. Path resolved from `app.path().app_config_dir()`.
+- Frontend ipc: `saveWorkspace` / `loadWorkspace` wrappers.
+- `tree.ts`: added `changeDistro` (with id swap to force XtermPane remount), `changeLabel`, and 5 preset trees (single, 2H, 3H, 2V, 2×2).
+- New `lib/layout/ops.ts` with `PaneOps` interface; refactored `Pane.svelte` / `SplitNode.svelte` / `LeafPane.svelte` to take `ops` instead of individual callbacks.
+- `LeafPane.svelte`: in-toolbar pane-label editor (click to rename, Enter saves, Esc cancels) and distro chip with click-popover. Picking a different distro in the popover respawns the pane.
+- `App.svelte`: migrated to APPDATA persistence with 500ms debounce. One-time localStorage→APPDATA migration on boot. Split inherits parent's distro+cwd via `findLeaf`. Titlebar preset buttons (1 / 2H / 3H / 2V / 2×2) with a confirm prompt when replacing >1 pane.
+- `pnpm check` clean (109 files, 0 errors, 0 warnings).
+- Manual verification on Windows: (to fill in)
+
 ### 2026-05-22 — M2 splits-tree layout
 
 - Added `src/lib/layout/`: `tree.ts` (pure helpers: types, newLeaf, splitLeaf, closeLeaf, replaceById, serialize/deserialize with shape-checking), `SplitNode.svelte` (flex container + draggable gutter with pointer-capture), `LeafPane.svelte` (toolbar with split-right/split-down/close + XtermPane underneath), `Pane.svelte` (recursive dispatcher).