fix(engine-manager): fail closed on unpinned downloads and escalate engine stop

[woodsonl]

Description

Unpinned downloads fail closed. download() executed any HTTPS artifact
whose manifest carried no sha256. A manifest without a digest means any host
that serves that URL yields code execution on the machine. It now returns an
error naming the computed digest, so the manifest author can pin it, and the
checksum-mismatch error names the same escape hatch so a stale pin is not a
dead end. Operators who accept unverified downloads can opt in with
NVPAIR_ALLOW_UNPINNED_DOWNLOADS=1. The bundled Ollama and LM Studio fetch
URLs are rolling, so their manifests carry no pin and install through that
opt-in; the check exists so a manifest can add one.

Engine stop escalates. stop() sent one graceful signal and then waited
forever, so an engine that ignored SIGTERM was never stopped. It now waits
the manifest's stop grace (default 5s) and escalates to a forced kill of the
whole process group or tree, which reaches engines that forked helper processes
such as model runners. Both waits are bounded, so a forced kill that itself
fails cannot hang shutdown either.

Release intent

Changelog title

Engine installs require a pinned digest, and stop escalates

Changelog body

Engine downloads without a pinned sha256 in their manifest are now refused by
default; an operator can opt in to unverified downloads explicitly. Stopping an
engine that ignores the graceful signal now escalates to a forced kill after
the manifest's grace period instead of waiting indefinitely.

Bumps

• services: patch
• nvpair-cluster-manager: none
• nvpair-engine-manager: patch
• nvpair-errors: none
• nvpair-job-scheduler: none
• nvpair-manual-nodes: none
• nvpair-node-info: none
• nvpair-node-scanner: none
• nvpair-node-settings: none
• nvpair-proxy: none
• nvpair-tui: none
• nvpair-ui-broker: none
• nvpair-workload-manager: none

Scope

Included: the download pin check and its opt-out, the stop escalation, the
manifest pins, and the tests for both.

Excluded: the install and stop commands themselves are unchanged apart from the
pin check and the added escalation step.

Validation

• go build ./... and go test ./... in services/nvpair-engine-manager
• services/nvpair-engine-manager/remediation_test.go and launch_test.go
  cover the changed install and stop paths.

Risk

Behavior change for operators with unpinned manifests: a download that used to
succeed now fails until the manifest is pinned or the opt-out is set. The
grace-period escalation force-kills an engine that has not exited; that is the
intended behavior and matches what Windows already did.

Checklist

• I have read the Contributing Guidelines.
• Every commit is signed off (git commit -s), certifying the Developer Certificate of Origin.
• New or existing tests cover the change.
• Relevant documentation is updated.
• I checked the diff, changed filenames, and commit messages for credentials, private data, internal URLs, internal issue identifiers, and generated artifacts.
• I recorded the validation commands and results above.
• I declared version bumps in the release-intent block above. services/versions.json is written by automation — do not edit it by hand.