fix: remediate dependency and supply-chain alerts

[jason-allen-oneal]

What changed

• refresh the lock graph to patched Starlette, idna, Pygments, click, and related current releases
• generate a hash-locked runtime requirements set and install it without mutable bootstrap upgrades
• pin the runtime and ClusterFuzzLite base images by digest
• hash-pin the Atheris fuzz dependency
• extract the client-error sanitizer into a lightweight module so the fuzzer packages and exercises the real production function

Alert coverage

This supersedes Dependabot PRs #6 and #12 and addresses open dependency alerts #8 and #10-#15. Alert #7 (DiskCache pickle deserialization) has no patched release and exists only through the optional embed extra; it is not installed in the default/runtime image and is not used by SignalGate source. That alert will receive a separate evidence-backed risk disposition.

The pinned-image and hash-locked changes address Scorecard PinnedDependencies alerts #2-#9. Governance alerts for repository age, historical review ratio, and external CII registration remain visible.

Validation

• uv lock check and frozen sync passed
• Ruff passed
• 38 non-E2E tests passed
• pip-audit 2.10.1 reported zero vulnerabilities for the runtime requirements
• runtime Docker build passed with hash enforcement
• Docker compose black-box suite passed
• packaged Atheris fuzzer completed 100 local runs

Pinned base: b8ea0de

[morrow-bluedot]

Independent exact-head review at 186cb09: patched locks, hash-locked runtime/fuzzer inputs, digest-pinned images, sanitizer extraction, Docker gates, CodeQL, and ClusterFuzz all pass. The hosted fuzzer completed 9,877,002 executions without a crash. DiskCache remains separately documented as no-fix optional-only risk.