Mesh/Sprite3d: blended (alpha) pass for soft-transparent 3D sprites

[obiot]

Context

The 3D mesh pass is opaque — MeshBatcher.bind() does gl.disable(gl.BLEND). Sprite3d (#1513) handles transparency via an alpha cutout (alphaCutoff, default 0.5), which is correct and cheap for characters/foliage/signs (crisp silhouette, correct depth, no sorting).

But it can't do smooth alpha blending — soft particles, glows, fades, additive FX. Those need a blended, depth-sorted pass.

Proposal

Add an opt-in blended path for meshes/Sprite3d:

• A blended draw mode (e.g. blend: true / a blend mode) that renders with BLEND enabled and depthMask(false) (depth-test on, depth-write off).
• Back-to-front ordering for the blended set (the existing depth sort can drive this), drawn after the opaque pass.
• Optional additive blend mode for glows.
• Per-sprite opacity/alpha fade (currently impossible since alpha is ignored).

Considerations

• Transparent ordering is inherently order-dependent; document the limitation (intersecting blended sprites won't resolve per-pixel).
• Keep the opaque + cutout path the default (no perf regression for the common case).
• WebGPU port shape should stay clean (the batcher already owns mesh-mode GL state).

Acceptance

• A semi-transparent Sprite3d (e.g. spherical particle) blends smoothly instead of hard-cutting.
• Per-sprite opacity fade works.
• Renders after opaque, sorted back-to-front; opaque/cutout path unchanged.

Refs: "soft alpha blending" out-of-scope note from #1513; Working in 3D transparency caveat.