Whiteboard: full design pass (feel, keybinds, effects, engine, persistence)

[worflor]

Outcome

The whiteboard becomes something you'd genuinely live in. This means a discovery and design pass on what actually feels good, then building that.

Where it stands

Built, uneven, under-tested (docs/STATUS.md). Notably, it can't save its ink to disk yet.

Candidate workstreams

1. Feel: stroke smoothing, pressure and velocity response, input latency
2. Keybinds: quick tool switch, undo/redo, colour and size, clear, show/hide
3. Effects: brush variety, shapes, highlighter, laser/fade, snap-to
4. Engine: does the current renderer scale, or does it need a rethink? (see the GPU overlay issue)
5. Persistence: save and load ink, and maybe export (the original ask)

Starting points

• crates/neuron-app/src/whiteboard.rs

Next step

Scope in Triage, then split into sub-issues.