fix(ci): allow cold Chrome startup in runtime fixtures

Let the fixture lifecycle own CDP readiness with the same 60-second startup deadline as the broader browser verifier. Return the discovered port to both consumers and clean up failed startup before returning a lease.

Validation: five startup/cleanup boundary tests, npm run check, and real Chrome 154 runtime and source-map CLI/MCP verifiers passed on macOS arm64. Hosted Linux verification follows the push.

scripts/lib/browser-nested-source-map-e2e.mjs

@@ -1,13 +1,11 @@
 import assert from "node:assert/strict";
 import { execFile } from "node:child_process";
 import { createHash } from "node:crypto";
-import { join } from "node:path";
 import { fileURLToPath } from "node:url";
 import { promisify } from "node:util";
 import { TraceMap, eachMapping } from "@jridgewell/trace-mapping";
 import { CdpBrowserProvider } from "../../dist/browser/CdpBrowserProvider.js";
 import { CdpConnection } from "../../dist/browser/CdpConnection.js";
-import { waitForBrowserDevtoolsPort } from "../../dist/browser/BrowserProcessStartup.js";
 import { parseEvidence } from "../../dist/domain/evidence.js";
 import { traceSourceMap } from "../../dist/javascript/sourceMaps/TraceSourceMap.js";
 import { startBrowserSourceMapSite } from "../fixtures/browser-source-map-site.mjs";
@@ -63,7 +61,6 @@ async function verifyCase(executable, entrypoint, scenario) {
   let fixture;
   let connection;
   let primaryError;
-  let stderr = "";
   let phase = "native-map-admission";
   let browserVersion;
   let expected;
@@ -90,17 +87,7 @@ async function verifyCase(executable, entrypoint, scenario) {
 
     phase = "owned-browser-startup";
     fixture = await startRuntimeFixtureBrowser(executable, site.origin);
-    fixture.browser.stderr.on("data", (chunk) => {
-      if (stderr.length < 65536) stderr += chunk;
-    });
-    const port = await waitForBrowserDevtoolsPort({
-      child: fixture.browser,
-      executable,
-      activePortPath: join(fixture.profile, "DevToolsActivePort"),
-      stderr: () => stderr,
-      timeoutMs: 20_000,
-    });
-    const endpoint = `http://127.0.0.1:${port}`;
+    const endpoint = `http://127.0.0.1:${fixture.port}`;
     const provider = new CdpBrowserProvider();
     const targets = await provider.listTargets({
       cdp_endpoint: endpoint,