feat(producer): add request-level render concurrency semaphore (#232)

Add a FIFO semaphore to limit concurrent renders in the producer server,
preventing Chrome CPU contention that causes beginFrame failures.

- New Semaphore utility class (packages/producer/src/utils/semaphore.ts)
- Both blocking render and SSE renderStream handlers acquire/release the semaphore
- SSE stream sends a "queued" event when request must wait
- New GET /render/queue endpoint exposes active/queued render counts
- Configurable via HandlerOptions.maxConcurrentRenders or PRODUCER_MAX_CONCURRENT_RENDERS env var (default: 2)
- New --max-concurrent-renders CLI flag (1-10)

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

packages/cli/src/commands/render.ts

@@ -95,6 +95,10 @@ export default defineCommand({
       description: "Fail render on lint errors AND warnings",
       default: false,
     },
+    "max-concurrent-renders": {
+      type: "string",
+      description: "Max concurrent renders when using the producer server (1-10). Default: 2.",
+    },
   },
   async run({ args }) {
     // ── Resolve project ────────────────────────────────────────────────────
@@ -135,6 +139,19 @@ export default defineCommand({
       workers = parsed;
     }
 
+    // ── Validate max-concurrent-renders ─────────────────────────────────
+    if (args["max-concurrent-renders"] != null) {
+      const parsed = parseInt(args["max-concurrent-renders"], 10);
+      if (isNaN(parsed) || parsed < 1 || parsed > 10) {
+        errorBox(
+          "Invalid max-concurrent-renders",
+          `Got "${args["max-concurrent-renders"]}". Must be a number between 1 and 10.`,
+        );
+        process.exit(1);
+      }
+      process.env.PRODUCER_MAX_CONCURRENT_RENDERS = String(parsed);
+    }
+
     // ── Resolve output path ───────────────────────────────────────────────
     const rendersDir = resolve("renders");
     const ext = FORMAT_EXT[format] ?? ".mp4";