ci: route heavy Rust jobs through Incredibuild build runners

.github/workflows/codspeed.yml

@@ -14,10 +14,33 @@ permissions:
 jobs:
   benchmarks:
     name: Run benchmarks
+    # Reverted from incredibuild-runner to ubuntu-latest after CI run
+    # 25722680967 reproducibly failed with:
+    #   setarch: failed to set personality to x86_64: Operation not permitted
+    #   ##[error]failed to execute valgrind
+    # The CodSpeedHQ action's `cargo codspeed run` step shells out to
+    # valgrind, which calls setarch to set ADDR_NO_RANDOMIZE personality.
+    # The IB self-hosted runner image runs under restricted Linux
+    # capabilities (no SYS_ADMIN, user-namespace remap), so the
+    # personality syscall is blocked. github-hosted runners allow it.
+    #
+    # Decision: keep the production CodSpeed workflow on ubuntu-latest.
+    # A hybrid "build on IB, run on ubuntu" flow would need fragile
+    # target-dir/artifact pinning across cargo-codspeed's instrumented
+    # outputs. The clean fix is runner-image support for setarch /
+    # personality(2); until then, CodSpeed stays on the runner that can
+    # execute Valgrind. The monty-side measurement of the IB-build value
+    # remains local in this repo via:
+    # ib-bench.yml::cell-I-ib-codspeed (which only does `cargo codspeed
+    # build`, no valgrind run, so it works on IB).
+    #
+    # If this workflow fails on ubuntu-latest with "Failed to retrieve
+    # upload data: 401 Unauthorized", that is CodSpeed auth / repository
+    # permissions, not an IB runner issue.
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           persist-credentials: false
 
@@ -45,7 +68,7 @@ jobs:
         run: cargo codspeed build -p monty-bench --bench main
 
       - name: Run benchmarks
-        uses: CodSpeedHQ/action@d872884a306dd4853acf0f584f4b706cf0cc72a2 # v4.13.0
+        uses: CodSpeedHQ/action@d872884a306dd4853acf0f584f4b706cf0cc72a2 # v4.13.0
         with:
           mode: simulation
           run: cargo codspeed run -p monty-bench --bench main