Align mesh base Y and remove legacy yOffset compensation

[tracygardner]

Motivation

• Stop using runtime yOffset compensation and make mesh placement geometry-consistent by aligning the mesh base to the requested Y once during setup (and again when deferred ground resolution occurs).

Description

• Add alignMeshBaseToY helper in api/mesh.js and call it during mesh initialization and after deferred ground-level resolution to align the wrapped mesh base to the requested Y.
• Remove writes to legacy metadata.yOffset in api/mesh.js so initialization no longer persists runtime Y compensation data.
• Simplify block/inspector Y handling by making calculateYPosition in ui/blocklyutil.js return mesh.position.y directly and rename the unused parameter to _block to avoid unused-variable warnings.
• Update gizmo drag-end behavior in ui/gizmos.js to use the direct calculated mesh Y and stop subtracting the bounding-box extent so block position mirrors the mesh transform.

Testing

• Ran npx eslint api/mesh.js ui/blocklyutil.js ui/gizmos.js; the run still fails due to multiple pre-existing lint issues unrelated to this change, though this PR removes the local unused-arg for calculateYPosition by renaming the parameter to _block.
• No additional automated functional tests were added in this change.

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Codex Task