Relay a cancelled job's SIGTERM to the build process

[jeremydmiller]

Closes the gap #4108's first real capped job exposed within hours of merging: CIKafka wedged on batch_processing_with_kafka.end_to_end, the new stall detector named the test and its pid in the log (STALLED: … has been in flight 300s on lane 0 (pid 3814)) — and the partial ledger never happened. The runner signals only the step's own shell on cancellation, and nothing in the bash → build.sh → dotnet run chain forwards SIGTERM to the grandchild where registerCancellationCapture lives.

The fix: a pid handshake

• registerCancellationCapture (build/StallCapture.cs) publishes Environment.ProcessId to .nuke/temp/build.pid when it arms.
• Workflow test steps run through build/run-with-cancellation-relay.sh, whose INT/TERM trap signals exactly that pid — deliberately not the process group, which would also kill the wedged worker the handler wants dotnet-dump to capture — then holds the step open (bounded) while the handler writes the ledger and takes stacks, inside the runner's own hard-kill deadline.
• Applied to tests.yml, http.yml, slow-tests.yml, and dotnet.yml's test steps. The smoke/pack steps keep plain ./build.sh — no supervisor, nothing to relay to.

Proven by simulating the runner locally

SIGTERM sent to the relay while CoreTests was mid-run:

[stall] relaying cancellation to the build process (pid 56144)
[stall] cancellation signal received — writing the partial ledger before the runner discards the run
=== CoreTests: PARTIAL — the run was still in flight when this view was taken
    533 passed, 2107 indeterminate …

and on disk: {"IsPartial": true, "CleanPasses": 533, "Indeterminate": 2107} — the 533 verdicts heard on the live stream survive, everything unverdicted is honestly Indeterminate, and the uploaded artifact is what outlives the cancelled job's discarded logs (#4098).

Also fixes a compile-time latent in #4111's sibling file: StallCapture.cs was missing using Nuke.Common.IO; for the AbsolutePath extensions the pid write needs.

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