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/.env.example

@@ -9,7 +9,10 @@ MINIO_USE_SSL=false
 MINIO_ACCESS_KEY=mcmapper
 MINIO_SECRET_KEY=changeme-set-in-untracked-env
 
-# cpu | gpu | hybrid — see render::backend. Only `cpu` exists so far (Phase 8 adds gpu/hybrid).
+# cpu | gpu | hybrid (Phase 8) — see render/mod.rs's RenderBackend doc comment. `gpu` offloads
+# tile shading and per-voxel face-visibility extraction to a wgpu compute shader; `hybrid` offloads
+# only tile shading and keeps meshing on CPU. Unrecognized values, and `gpu`/`hybrid` on a machine
+# with no compatible GPU adapter, fall back to `cpu` with a logged warning rather than crashing.
 RENDER_BACKEND=cpu
 
 # Thread count for the rayon pool that renders a batch of dirty chunks in parallel (see