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.

api/src/ws-gateway.ts

@@ -1,6 +1,6 @@
 import { eq } from "drizzle-orm";
 import { db } from "./db/client";
-import { chunkColumns, servers } from "./db/schema";
+import { chunkColumns, chunkSections, servers } from "./db/schema";
 import { markChunkDirty } from "./redis";
 
 // Wire protocol (mod <-> api), one JSON object per WS text frame:
@@ -10,13 +10,21 @@ import { markChunkDirty } from "./redis";
 //               {"type":"hello_ack","ok":false,"error":"..."}  (connection closed after)
 //
 //   mod -> api  {"type":"columns","dimension":0,"columns":[{"x":..,"z":..,"height":..,"blockId":..,"blockMeta":..}]}
+//   mod -> api  {"type":"sections","dimension":0,"chunkX":..,"chunkZ":..,"sections":[{"sectionY":4,"blocks":"<base64>"}]}
 //
 // `columns` doubles as both initial backfill (one message per loaded chunk, 256 columns) and
 // live deltas (one message per flush tick, just the columns that changed) — both are just "here
 // is the current topmost block + height for these XZ columns", the mod recomputes it from its
 // own world access rather than the api trying to infer a post-break top block from a raw diff.
-// Full per-voxel data (needed for Phase 2 3D meshing) is a natural extension of this same
-// connection once the chunk store grows a full block-data column.
+//
+// `sections` is the Phase 2 addition for full-voxel 3D meshing, additive to `columns` (see
+// db/schema.ts's chunkColumns/chunkSections comments) — one message per loaded chunk at load
+// time (all non-empty 16x16x16 sections), and again at flush time for chunks touched since the
+// last flush (the whole section is resent, same "current state, not a diff" philosophy as
+// columns — see DeltaEvent's javadoc on the mod side). `blocks` is 4096 little-endian u16
+// blockStateIds, base64-encoded, indexed by `(ly*16 + lz)*16 + lx` within the section.
+// A "sections" message marks the chunk dirty the same way "columns" does — one dirty-chunk
+// event now triggers the worker to re-render both the 2D tile and any 3D meshes for that chunk.
 
 interface Column {
   x: number;
@@ -26,6 +34,11 @@ interface Column {
   blockMeta: number;
 }
 
+interface Section {
+  sectionY: number;
+  blocks: string;
+}
+
 interface ConnState {
   serverId: string;
 }
@@ -107,6 +120,40 @@ export const wsGateway = {
       }
       return;
     }
+
+    if (msg.type === "sections") {
+      const dimension: number = msg.dimension;
+      const chunkX: number = msg.chunkX;
+      const chunkZ: number = msg.chunkZ;
+      const sections: Section[] = msg.sections ?? [];
+      if (sections.length === 0) return;
+
+      await db
+        .insert(chunkSections)
+        .values(
+          sections.map((s) => ({
+            serverId: state.serverId,
+            dimension,
+            x: chunkX,
+            z: chunkZ,
+            sectionY: s.sectionY,
+            blocks: s.blocks,
+          })),
+        )
+        .onConflictDoUpdate({
+          target: [
+            chunkSections.serverId,
+            chunkSections.dimension,
+            chunkSections.x,
+            chunkSections.z,
+            chunkSections.sectionY,
+          ],
+          set: { blocks: sqlExcluded("blocks"), updatedAt: new Date() },
+        });
+
+      await markChunkDirty(state.serverId, dimension, chunkX, chunkZ);
+      return;
+    }
   },
 
   close(ws: any) {