Phase 0: scaffold three-service backend (api/worker/frontend)

api/ (ElysiaJS+Bun WS gateway skeleton), worker/ (Rust, Redis dirty-chunk
stream consumer behind a swappable RenderBackend trait), frontend/
(ElysiaJS+Pug+Tailwind4+Alpine, one server-rendered page) — all three
verified running locally. docker-compose wires them up with postgres,
redis, minio (rendered tile/mesh object storage), and Caddy as reverse
proxy.

worker/src/render/mod.rs

@@ -0,0 +1,14 @@
+mod cpu;
+
+pub use cpu::CpuRenderBackend;
+
+/// Swappable rendering strategy (CPU via rayon / GPU via wgpu / hybrid) selected at startup
+/// via the `RENDER_BACKEND` env var. Only the CPU path exists so far; GPU/hybrid land in
+/// Phase 8, with automatic fallback to CPU if `gpu`/`hybrid` is requested but no compatible
+/// GPU is found.
+pub trait RenderBackend: Send + Sync {
+    fn name(&self) -> &'static str;
+
+    // Tile rasterization and chunk meshing methods land alongside their Phase 1/2 callers —
+    // no point defining signatures for data shapes that don't exist yet.
+}