Replace the legacy held-Alt mechanism on wall and fence drafting with a
mode toggle, mirroring the snapping-mode chip:
- Wall: `wallChainMode` room (auto-close on loop) / single. Room mode
finishes automatically when the new endpoint lands within the join-snap
radius of the chain's first vertex; single commits one wall per click.
- Fence: `fenceChainMode` continuous (chain until double-click/Esc) /
single. Fences are linear barriers, so continuous has no auto-close.
- Both: Alt-tap cycles the active drafting tool's chain mode (clean-tap,
scoped to wall/fence drafting); a clickable HUD chip shows the mode.
Persisted + migrated in `useEditor`.
Migrate wall and fence off held-Alt-bypass-alignment to the unified
convention: alignment now follows the magnetic snap mode, which frees Alt
for the toggle. 2D floorplan parity kept in sync with the 3D tools.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
