ci: give the Ubuntu workspace test build disk and memory headroom

On every wave/0.10.1-next PR run, Test (ubuntu-latest) was terminated about
4.5 minutes into compiling the workspace test targets ("The runner has
received a shutdown signal", exit 143) at 04:30, 05:58, 06:02 and 07:26 UTC.
The 07:26 kill had no newer push to cancel it, so it was not the PR
concurrency group. Single-fix PRs finish the same --all-features build in
about 10 minutes; the combined wave is the largest tree.

For heavy Ubuntu PR/dispatch runs, add a step that removes preinstalled SDKs
the job never uses (dotnet, Android, GHC, CodeQL bundle) and adds 8 GiB of
swap, printing df/free before and after. The Run tests step now starts a
30-second memory/swap/disk trace on Linux and stops it after nextest,
preserving nextest's exit status, so a further termination records whether
memory or disk ran out. macOS and Windows are unchanged.

Validation: actionlint reports the same 12 pre-existing findings before and
after (none on the new lines); node .github/scripts/release-workflows.test.js
passes. Hosted proof is the next #6782 Ubuntu run.

Refs #6698

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Signed-off-by: CodeWhale Bot <[email protected]>

.github/workflows/ci.yml

@@ -822,6 +822,25 @@ jobs:
             sleep 15
           done
           sudo apt-get install -y libdbus-1-dev pkg-config
+      - name: Ubuntu build headroom
+        # The combined wave/0.10.1-next workspace build was terminated
+        # mid-compile on every PR run ("runner has received a shutdown
+        # signal", exit 143, no newer push to cancel it) while single-fix PRs
+        # finished the same build in ~10 minutes. Drop preinstalled SDKs this
+        # job never uses and add swap so the full --all-features test build
+        # has disk and memory headroom; the Run tests step logs both.
+        if: needs.changes.outputs.heavy == 'true' && matrix.os == 'ubuntu-latest' && (github.event_name == 'workflow_dispatch' || github.event_name == 'pull_request')
+        shell: bash
+        run: |
+          df -h / /mnt; free -m
+          sudo rm -rf /usr/share/dotnet /usr/local/lib/android /opt/ghc /usr/local/.ghcup /opt/hostedtoolcache/CodeQL
+          if sudo fallocate -l 8G /mnt/cw-swapfile && sudo chmod 600 /mnt/cw-swapfile \
+            && sudo mkswap /mnt/cw-swapfile >/dev/null && sudo swapon /mnt/cw-swapfile; then
+            echo "Added 8 GiB swap at /mnt/cw-swapfile"
+          else
+            echo "::warning::Could not add swap; continuing with the runner default"
+          fi
+          df -h / /mnt; free -m
       - uses: Swatinem/rust-cache@v2
         if: needs.changes.outputs.heavy == 'true' && (matrix.os != 'ubuntu-latest' || github.event_name == 'workflow_dispatch' || github.event_name == 'pull_request')
         with:
@@ -879,7 +898,20 @@ jobs:
         # one. nextest does not run doctests — the next step keeps them.
         if: needs.changes.outputs.heavy == 'true' && (matrix.os != 'ubuntu-latest' || github.event_name == 'workflow_dispatch' || github.event_name == 'pull_request') && !(github.event_name == 'workflow_dispatch' && inputs.workspace_test_mode == 'shared-process-twice')
         shell: bash
-        run: sh scripts/with-hermetic-test-home.sh cargo nextest run --workspace --all-features --locked --profile ci
+        run: |
+          monitor=
+          if [ "$RUNNER_OS" = Linux ]; then
+            # Headroom trace: if the runner is terminated again, the last lines
+            # say whether memory or disk ran out.
+            ( while sleep 30; do
+                echo "[headroom] $(free -m | awk '/^Mem:/{print "mem used " $3 "/" $2 " MiB"} /^Swap:/{print "swap used " $3 "/" $2 " MiB"}' | paste -sd ' ') disk free $(df -h / | awk 'NR==2{print $4}')"
+              done ) &
+            monitor=$!
+          fi
+          status=0
+          sh scripts/with-hermetic-test-home.sh cargo nextest run --workspace --all-features --locked --profile ci || status=$?
+          if [ -n "$monitor" ]; then kill "$monitor" 2>/dev/null || true; fi
+          exit "$status"
         env:
           CODEWHALE_EXT_HOST_TESTS: '1'
           # sccache 0.17 panics resolving its config directory under the