Cover public API edge cases - #172

Hi @mzl2233, thanks for picking this up! A few things before I can merge:

1. The Shape.ts change removes the immutable_midstep guard on sensorEnabled

That guard is intentional — it's one of ~30 similar mid-step guards across the engine, all there to prevent inconsistent arbiter state. I ran a quick stress test on your branch:

// PreListener toggles wallShape.sensorEnabled = true
// Result on your branch:
// - COLLISION callback still fires (should be SENSOR)
// - visitor velocity 120 → 0 (collision impulse applied — sensors shouldn't impart impulse)
// - SENSOR callback fires too, but a step later

Could you revert the Shape.ts change and rewrite the sensor-toggle test to validate the intended behaviour, which is that it throws?

expect(() => { wallShape.sensorEnabled = true; }).toThrow(/cannotbesetduringaspacestep/);

If you want to actually support mid-step sensor toggling, that's a separate design discussion (would need arbiter destruction + recreation, or a deferred change queue) — let's keep it out of this PR.

2. The PR covers 3 of the 10 cases listed in #170

That's totally fine if the PR scope is narrower than the issue, but in that case could you either expand the coverage or update the PR title/description to reflect what's actually in scope? Here's where we stand:

3. Housekeeping

• Please rebase onto current master (PR Feat/convex cast demo #156 landed since you forked).
• Run npm run format:check — there's a small Prettier issue in the test file.
• Add Closes #170 to the PR description so GitHub links the PR to the issue.

The body self-removal test and broadphase filter test look great. Thanks!