ci(mutation): cap Stryker concurrency at 6 to stop self-hosted host OOM

The metadata mutation gate (conformance:mutation, run by the ts-fast lane) set no
`concurrency`, so Stryker defaulted to cpuCount-1 = 23 test-runner workers on the
24-thread self-hosted host. Co-scheduled with the other heavy CI lanes (java-slow,
csharp, ...) that saturated the box and fired the kernel OOM killer. Capping at 6
leaves ~18 threads for the sibling lanes while keeping the gate fast.

Validated: mutation score 51.81 (>= break threshold 35), completes in ~2 min with
6 workers. The local-ci.yml ts-fast comment already flags this gate as flaky under
runner contention; this removes the root cause.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XeGSV3StPCcJGZNNJ4ZfAb

Author: dmealing

server/typescript/packages/metadata/stryker.conf.mjs

@@ -21,6 +21,11 @@ export default {
     "src/overlay.ts",
     "src/provider.ts",
   ],
+  // Cap parallelism. Stryker defaults to (cpuCount - 1) test-runner workers — 23
+  // on the 24-thread self-hosted host — which, co-scheduled with the other heavy
+  // CI lanes (java-slow, csharp, ...), starved the box into OOM. 6 leaves ~18
+  // threads for the sibling jobs while keeping the mutation gate reasonably fast.
+  concurrency: 6,
   thresholds: { high: 60, low: 40, break: 35 },
   reporters: ["clear-text", "html"],
 };