fix(nodes): wall split and rectangle feedback from the first QA round

[AxiomeCG]

What does this PR do?

Follow-up to #902 with the fixes from its first QA round, which landed a minute after the squash merge.

• One wheel notch is one cut. The split's cut count stepped every 60 px of wheel travel, and a notched wheel on macOS (e.g. an MX Master in ratchet mode) reports only a few pixels per notch, so it took three or four notches to change the count. A wheel event that follows an 80 ms pause now steps at once, and line-mode events always do. A continuous trackpad stream still steps by travel (packages/nodes/src/wall/split-pointer.ts).
• Split and merge make a sound. Committing a split, and a merge, plays the wall-placement sound (sfx:structure-build).
• The rectangle draft ticks like the line draft. Moving the second corner plays sfx:grid-snap once per snapped position, and the first corner plays the line tool's start sound, in 3D and in 2D.
• The wall tool keeps its last shape. Leaving the wall tool used to reset Shape to Line; re-arming it after drawing rectangles now resumes Rectangle (for the session).

How to test

1. bun dev, select a wall, click Split (scissors). Scroll with a notched mouse wheel: each notch changes the cut count by one. A trackpad swipe still changes it smoothly by distance.
2. Click to commit the split: the wall-placement sound plays. Select the two halves and Merge: same sound.
3. Wall tool, press R (Shape: Rectangle), click the first corner (start sound), then move the mouse: a tick at each new snapped point, as in line mode. Check in 3D and in the 2D plan.
4. With Shape: Rectangle, switch to select mode, then pick the wall tool again: it comes back in Rectangle.
5. bun test in packages/nodes (new tests cover notched-wheel and trackpad stepping), bun check.

Screenshots / screen recording

Mostly feel and sound; a short recording of the notched wheel and the rectangle ticks will be added by the author if useful.

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch

🤖 Generated with Claude Code

https://claude.ai/code/session_017sG15rKXusC8rbBg6gjSRm

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Note

Low Risk
Localized UX changes to wall tools, pointer wheel handling, and optional SFX with no auth, data, or core scene-model changes beyond existing split/merge paths.

Overview
QA follow-up for wall split, merge, rectangle drafting, and line/rectangle shape memory.

Wall split scroll now treats each notched-wheel tick as one cut (gesture gap + line-mode deltas), while continuous trackpad scrolling still steps by accumulated travel. Tests cover notched wheels and trackpad streams.

Audio feedback adds sfx:structure-build on split commit and wall merge, and aligns rectangle drafting with the line tool: start sound on the first corner and sfx:grid-snap when the opposite corner moves to a new snapped position (3D and floor plan).

Shape mode no longer resets to Line when the wall tool unmounts; re-selecting the wall tool keeps Rectangle (or Line) for the session.

^{Reviewed by Cursor Bugbot for commit 9ef12de. Bugbot is set up for automated code reviews on this repo. Configure here.}

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