Phase 38: Universal API skeleton — /v1/chat, /v1/usage, /v1/sessions

First slice of the control-plane pivot. OpenAI-compatible surface
over the existing aibridge → Ollama path. Additive — no existing
routes touched. All 7 unit tests green, release build clean.

What ships:
- crates/gateway/src/v1/mod.rs — router, V1State (ai_client + Usage
  counter), ChatRequest/ChatResponse/Message/UsageBlock types, handlers
  for /chat, /usage, /sessions. OpenAI-compatible field shapes:
  {model, messages[{role,content}], temperature?, max_tokens?, stream?}
- crates/gateway/src/v1/ollama.rs — shape adapter. Flattens messages
  into (system, prompt), calls aibridge.generate, unwraps response
  back into OpenAI /v1/chat shape. Prefers sidecar-reported tokens;
  falls back to chars/4 ceiling estimate matching Phase 21 convention.
- crates/gateway/src/main.rs — one new mod, one .nest("/v1", ...)

Tests (7/7):
- chat_request_parses_openai_shape
- chat_request_accepts_minimal
- usage_counter_default_is_zero
- flatten_separates_system_from_turns
- flatten_concatenates_multiple_system_messages
- flatten_with_no_system_returns_empty_system
- estimate_tokens_chars_div_4_ceiling

Not in this phase (per CONTROL_PLANE_PRD.md): streaming, tool calls,
session state, multi-provider, fallback chain, cost gating. All
land in Phases 39-44.

Next: live-test POST /v1/chat after gateway restart, then migrate
bot/propose.ts off direct sidecar calls to prove the loop end-to-end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/gateway/src/main.rs

@@ -3,6 +3,7 @@ mod access_service;
 mod auth;
 mod observability;
 mod tools;
+mod v1;
 
 use axum::{Router, extract::DefaultBodyLimit, routing::get};
 use proto::lakehouse::catalog_service_server::CatalogServiceServer;
@@ -183,6 +184,14 @@ async fn main() {
                 registry: tool_reg,
                 query_fn: tools::QueryExecutor::new(engine.clone()),
             }
+        }))
+        // Phase 38 — Universal API skeleton. Thin OpenAI-compatible
+        // surface over the existing aibridge → Ollama path. Future
+        // phases add provider adapters (39), routing engine (40),
+        // session state (41), etc. All without changing this mount.
+        .nest("/v1", v1::router(v1::V1State {
+            ai_client: ai_client.clone(),
+            usage: std::sync::Arc::new(tokio::sync::RwLock::new(v1::Usage::default())),
         }));
 
     // Auth middleware (if enabled)