fix: unblock release guard pack benchmarks

[RyshMan]

Problem
The release hotfix branch was blocked in pre-tag validation because the guard pack benchmarks regressed past the make bench-check envelope, specifically BenchmarkVerifyPackTypical and BenchmarkBuildIncidentPackTypical.

Changes

• reuse the already-loaded runpack when building incident packs so the incident path does not reread the same artifact
• add a small-pack verify fast path that opens tiny evidence zips in memory
• index small pack entries earlier in verification to cut repeated lookup cost
• annotate the new local artifact read for the existing gosec policy

Validation

• gait doctor --json
• make prepush-full
• targeted benchmark reruns during remediation
• make bench-check
• make test-v2-4-acceptance
• make test-release-smoke