Phase 0: bootstrap Rust workspace

- Cargo workspace with 6 crates: shared, storaged, catalogd, queryd, aibridge, gateway
- shared: types (DatasetId, ObjectRef, SchemaFingerprint, DatasetManifest) + error enum
- gateway: Axum HTTP entrypoint with nested service routers + tracing
- All services expose /health stubs
- justfile with build/test/run recipes
- PRD, phase tracker, and ADR docs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/aibridge/src/lib.rs

@@ -0,0 +1 @@
+pub mod service;

crates/aibridge/src/service.rs

@@ -0,0 +1,9 @@
+use axum::{Router, routing::get};
+
+pub fn router() -> Router {
+    Router::new().route("/health", get(health))
+}
+
+async fn health() -> &'static str {
+    "aibridge ok"
+}