Support the z coordinate when following spatial audio sources

[ICOM725]

This PR

• Adds a new feature

Describe the changes below:

Follow the source object's optional z coordinate when updating a spatial sound, including zero and negative values. Keep the existing depth when z is absent or undefined, and document that follow accepts a Vector3Like object.

Add regression coverage for changing coordinates, zero depth, two-dimensional targets, undefined depth, paused playback and unchanged coordinates. This change is limited to following a source; it does not add a new WebAudioSound.z accessor.

Fixes #7135.

Validation:

• Before the implementation, the initial five new tests produced three failures and two passing compatibility checks.
• Sound tests: 6 files, 378 tests passed, including the final six new tests.
• ESLint 8.57.1 passed for the three changed files with the repository configuration, test globals and lines-around-comment disabled. Without that override, the existing test at line 321 fails the same rule on both the baseline and this branch.
• git diff --check passed.

The JSDoc -X check could not complete because the installed JSDoc 3 calls util.isRegExp, which is unavailable in Node 25. No dependency changes or generated files are included.