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.

frontend/src/views/index.pug

@@ -14,6 +14,7 @@ html(lang="en")
     div#app.h-screen.flex.flex-col(x-data="mapmapper()" x-init="init()")
       header.px-4.py-2.flex.items-center.gap-4.border-b.border-neutral-700
         h1.text-lg.font-semibold MCMapper
+        a.text-sm.text-neutral-400.underline(href="/3d") 3D view
         span.text-sm.text-neutral-400(x-show="!loading && server")
           | Viewing:
           span(x-text="server?.name")

frontend/src/views/scene3d.pug

@@ -0,0 +1,18 @@
+doctype html
+html(lang="en")
+  head
+    meta(charset="utf-8")
+    meta(name="viewport" content="width=device-width, initial-scale=1")
+    title MCMapper — 3D
+    link(rel="stylesheet" href="/css/tailwind.css")
+    script(src="https://cdn.babylonjs.com/babylon.js")
+    style.
+      html, body, #renderCanvas { height: 100%; margin: 0; touch-action: none; outline: none; }
+  body.bg-neutral-900.text-neutral-100
+    div.h-screen.flex.flex-col
+      header.px-4.py-2.flex.items-center.gap-4.border-b.border-neutral-700
+        h1.text-lg.font-semibold MCMapper — 3D
+        a.text-sm.text-neutral-400.underline(href="/") back to 2D map
+        span#status.text-sm.text-neutral-500 loading…
+      canvas#renderCanvas.flex-1
+    script(src="/js/mesh.js")