layout simplification (#2387)

The current layout system uses a complex bidirectional atom architecture
that forces every layout change to round-trip through the backend
WaveObject, even though **the backend never reads this data** - it only
queues actions via `PendingBackendActions`. By switching to a "write
cache" pattern where local atoms are the source of truth and backend
writes are fire-and-forget, we can eliminate ~70% of the complexity
while maintaining full persistence.

----

Every layout change (split, close, focus, magnify) currently follows
this flow:

```
User action
  ↓
treeReducer() mutates layoutState
  ↓
layoutState.generation++  ← Only purpose: trigger the write
  ↓
Bidirectional atom setter (checks generation)
  ↓
Write to WaveObject {rootnode, focusednodeid, magnifiednodeid}
  ↓
WaveObject update notification
  ↓
Bidirectional atom getter runs
  ↓
ALL dependent atoms recalculate (every isFocused, etc.)
  ↓
React re-renders with updated state
```

---

## Proposed "Write Cache" Architecture

### Core Concept

```
User action
  ↓
Update LOCAL atom (immediate, synchronous)
  ↓
React re-renders (single tick, all atoms see new state)
  ↓
[async, fire-and-forget] Persist to WaveObject
```

### Key Principles

1. **Local atoms are source of truth** during runtime
2. **WaveObject is persistence layer** only (read on init, write async)
3. **Backend actions still work** via `PendingBackendActions`
4. **No generation tracking needed** (no need to trigger writes)

Author: sawka

.gitignore

@@ -16,6 +16,7 @@ out/
 make/
 artifacts/
 mikework/
+.env
 
 # Yarn Modern
 .pnp.*