Fix asymmetric mujoco-assets/ prefix matching in buildZipCandidates

[midasbal]

What

buildZipCandidates in src/lib/mujoco-asset-loader.ts matched asset paths
asymmetrically across the two asset-prefix flavors. A reference using the
hyphen prefix (mujoco-assets/) never generated the underscore candidate
(mujoco_assets/<stem>), so an asset stored in the zip under the underscore
prefix was missed. It also emitted useless doubled-prefix candidates such as
mujoco-assets/mujoco-assets/....

Why it matters

When the XML references mujoco-assets/... and the zip stores
mujoco_assets/..., the zip lookup misses. With zipOnly enabled this throws
Asset not found in zip and the asset fails to load; otherwise it silently
falls back to an individual fetch.

Fix

Derive a single prefix-agnostic stem (strip whichever known prefix is present)
and build candidates from it: normalized, stem, mujoco-assets/<stem>,
mujoco_assets/<stem>. Matching is now symmetric.

Verification

• mujoco-assets/tex/wood.png ref vs mujoco_assets/tex/wood.png entry: now matches (was a miss).
• mujoco_assets/... ref vs mujoco-assets/... entry: still matches.
• Bare-stem ref matches either stored prefix, and vice versa.
• An unrelated asset still does not match.

Notes

Scoped to the one function; no dependencies or config changed. The repo has no
test runner, so I did not add one here. Happy to add a regression test covering
both prefix directions if you'd like to adopt one.