Fixed #31550

Description

mergeVertices() used bitwise coercion to truncate each quantized attribute value. Bitwise operations first wrap Numbers to signed 32-bit integers, so a zero or very small tolerance can make distinct vertices collide in the hash.

This replaces the coercion with Math.trunc(), which preserves the full Number range, and adds a regression with positions [0, 8, 0] at zero tolerance. The repeated origin is merged while the vertex at x=8 remains distinct.

Validation:

• Focused runtime check returned two positions and index [0, 1, 0].
• git diff --check passed.
• The full QUnit suite was not available because the local dependency cache was incomplete and the configured package registry was unreachable.