Add AC-1/AC-2 CI with golden fixture comparison

[sehkone]

Problem

AC-0 pipeline CI (#24) catches basic regressions, but AC-1/AC-2 integration and golden fixture comparison are needed to catch subtler issues across the full scope.

Scope

• CI workflow that runs AC-1 and AC-2 scenarios (manual trigger or nightly schedule)
• Scenarios must set a fixed start_at (per Add scenario start time and time compression with timestamp rewriting #67) so generated bundles are deterministic across CI runs
• Compare generated bundles against golden fixtures from Add golden fixtures and deterministic regression checks #23
• Execute a lightweight QFlow replay smoke test (if the contract from Define QFlow replay contract and smoke-test interface #26 is ready) or a placeholder check
• Pre-baked container images from Build and publish pre-baked container images for generator environments #22 for faster CI runs

Out of Scope

• AC-0 basic CI (already covered by Add AC-0 pipeline CI #24)
• Windows VM provisioning in CI (requires nested virtualization)
• Performance benchmarking

Done when

• AC-1/AC-2 can be triggered manually or on a schedule
• Golden fixture comparison catches schema, path, and Ground Truth regressions
• Build fails on Validator errors or fixture drift

Dependencies

• Build and publish pre-baked container images for generator environments #22 (pre-baked images for fast CI runs)
• Add golden fixtures and deterministic regression checks #23 (golden fixtures for comparison)
• Run AC-1 and AC-2 end-to-end and fix integration gaps #21 (AC-1 and AC-2 must be stable)
• Add scenario start time and time compression with timestamp rewriting #67 (fixed start_at and timestamp rewriting — required for fixture determinism)