Add GPU-accelerated rendering: cpu/gpu/hybrid RenderBackend (Phase 8)

Splits RenderBackend's per-voxel face-visibility extraction and tile
shading out as GPU-offloadable steps (wgpu compute shaders), while
keeping greedy-mesh merge/compaction CPU-only per the plan's "partial
GPU rendering" design. RENDER_BACKEND=cpu|gpu|hybrid selects the
strategy, falling back to cpu automatically (logged) if no compatible
GPU adapter is found. Verified against a real GPU: all tests pass,
including ones asserting byte-identical output between the cpu and
gpu backends; a new benchmark example honestly shows cpu currently
outperforming gpu/hybrid at realistic batch sizes since each call is
its own dispatch/readback round trip rather than batched across a
whole render batch (documented as a follow-up optimization).

Also fixes two real, pre-existing gaps found while validating the
worker's actual `docker build`: a missing .dockerignore was sending
the local multi-GB target/ dir into the build context, and the
Dockerfile's rust:1.80 pin was already too old for current
transitive dependency MSRVs (bumped to rust:1.97).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015tKdPZt78zbPUZMXWzKEKt

worker/.dockerignore

@@ -0,0 +1,2 @@
+target/
+.env