Phase 2: full-voxel chunk storage, greedy mesher, and Babylon 3D viewer

api: chunk_sections table (per 16x16x16 section, base64-encoded u16
blockStateId array) and mesh_pointers table, additive to Phase 1's
column-based chunk_columns/tile_pointers — 2D tile rendering keeps using
the cheap column path unchanged. New "sections" WS message (backfill on
chunk load + delta resend on flush, same "current state, not a diff"
philosophy as columns) reuses the existing dirty-chunk Redis event, so one
event now triggers the worker to re-render both the 2D tile and any 3D
meshes for that chunk. New mesh-serving routes.

worker: a from-scratch greedy mesher (per-axis 2D mask sweep + rectangle
merge — the standard voxel-meshing technique, reimplemented from its
public description, not copied from any codebase) producing a compact
custom binary vertex buffer per non-empty section. Verified with unit
tests, including one that specifically checks a uniform section collapses
to exactly 6 merged quads rather than one quad per voxel face (the
decisive signal that merging, not just per-voxel face emission, is
actually happening).

frontend: a barebones Babylon.js 3D viewer (/3d) that loads a fixed radius
of chunks, parses the mesh binary format, and renders each section as its
own mesh (no cross-section merging yet, no camera-based streaming yet —
both reasonable follow-ups once there's a reason to optimize).

End-to-end verified against live containers, including through the real
mod-side Java WS client (see MCMapper-Mod's matching commit): a known
half-solid section correctly round-trips to exactly 24 vertices / 36
indices at the mesh-serving endpoint, matching the "6 merged outer faces"
the unit tests predict.

worker/Cargo.lock

@@ -1732,6 +1732,7 @@ dependencies = [
  "aws-config",
  "aws-credential-types",
  "aws-sdk-s3",
+ "base64",
  "image",
  "rayon",
  "redis",