Phase 11: real block textures for 2D tiles

Layers texture-averaged colors on top of palette.rs's hand-picked table as a
fallback (color_for_textured), not a hard replacement — biome-tinted/animated
blocks (grass top, leaves, water, lava) deliberately keep the hand-picked
color since averaging their raw jar textures would be wrong, not just
imprecise.

Vanilla: worker/src/textures.rs downloads Mojang's official client jar
directly from launchermeta/piston-meta/piston-data (same endpoints the real
launcher uses, gated behind ACCEPT_MINECRAFT_EULA=true, off by default,
mirrors BlueMap's accept-download) and averages every block texture, cached
to disk so it's not re-downloaded every restart.

Modded: ingests block_registry/block_textures messages from the mod (new
Postgres tables + MinIO storage in api/src/textures.ts) — sent once per
connection over the existing WS gateway.

texturepacks/<name>/ (Dynmap-style flat PNGs) lets an operator override the
vanilla defaults worker-wide via TEXTURE_PACK. A per-server texturePack
admin column/API exists for the same purpose, but render-time per-server
resolution (of both the admin selection and the ingested modded textures) is
explicitly deferred — the worker still applies one process-wide palette; true
per-server resolution needs the render pipeline to thread a server-scoped
palette through the batch/GPU-dispatch path, judged too big a change for this
phase. Documented in README.

Docker was unavailable in this dev environment for the usual integration-test
verification; bunx tsc --noEmit was used as a fallback static check instead
(clean except 2 pre-existing unrelated errors in markers.test.ts).

e2e/tests/admin.spec.ts

@@ -47,6 +47,7 @@ test.describe("admin panel", () => {
     const row = page.locator('[data-testid="admin-server-row"][data-server-name="e2e-server"]');
     await row.getByTestId("admin-server-waypointformat").selectOption("xaero");
     await row.getByTestId("admin-server-anonchat").uncheck();
+    await row.getByTestId("admin-server-texturepack").fill("my-resource-pack");
     await row.getByTestId("admin-server-save").click();
     await expect(row.getByTestId("admin-server-status")).toHaveText("saved");
 
@@ -56,6 +57,7 @@ test.describe("admin panel", () => {
     const reloadedRow = page.locator('[data-testid="admin-server-row"][data-server-name="e2e-server"]');
     await expect(reloadedRow.getByTestId("admin-server-waypointformat")).toHaveValue("xaero");
     await expect(reloadedRow.getByTestId("admin-server-anonchat")).not.toBeChecked();
+    await expect(reloadedRow.getByTestId("admin-server-texturepack")).toHaveValue("my-resource-pack");
   });
 
   test("deleting a server removes it from the list", async ({ page }) => {