fix(garm): exact identity and dispatch occupancy for v0.2.1-nddev.94

[rldyourmnd]

Summary

• Bind stale scale-set job mutation to check-run external_id, check_run_url path ID, repository, attempt, numeric job ID and SHA. A name is not identity. Omitted or changing page totals retain the intent.
• Scaling uses live MESSAGE TotalAssignedJobs; idle retirement needs a recent messageID greater than 0. Replay of the same session/message and late messages from a replaced session do not refresh idle freshness. Session-create zeros and 202/nil long-polls are not idle evidence. Start failure deletes the new session and cancels the listener context.
• JobStarted of another GUID does not delete or rename an assigned waiter. A request-less JobAssigned yields occupancy when a different GUID becomes JobAvailable, including while still unexpired; the original waiter and FIFO stay. Same-GUID JobAvailable keeps occupancy. Listener JobCompleted is not REST workflow-job terminal.
• Official build stops the container before deleting the bind-mounted source. Derivative v0.2.1-nddev.94, binary SHA-256 44721568b2f8839e643a1976414de56a41e453854b5f41467a2cf2775b99bdad, Go 1.26.6, glibc 2.34. Published .92/.93 patches and the .93 artifact 8f6519500e90ad028c62b7fc3f5c8f2c7784fd82ef653c37c8db9e45afcfa841 are unchanged. Source/artifact candidate only; not a fleet rollout.

Test plan

• go test -tags testing -race -count=1 ./runner/pool ./workers/scaleset ./util/github/scalesets on the patched tree with Go 1.26.6 vendor
• Official scripts/build-garm-nddev.sh two-cache path twice; second run matches pin 44721568b2f8839e643a1976414de56a41e453854b5f41467a2cf2775b99bdad
• make verify with Go 1.26.7
• gds generate repository --check against immutable 0.8.0 bundle: no projection drift
• govulncheck v1.6.0 source-mode ./cmd/garm on the prior frozen tree: 0 called, 0 imported, 27 module-only; go.mod unchanged in this follow-up
• Coordinator review, private pin, runtime adoption — out of this PR