ci: shorten PR feedback without reducing verification coverage

[Brad-Edwards]

Problem

PR feedback is slower than necessary when independent checks are serialized or when small changes must wait for the broadest verification suite before receiving actionable test and static-analysis results. Comprehensive verification should remain; the improvement is to shorten the critical path and surface failures earlier.

Goal

Reduce time to first actionable PR feedback and final required-check completion without weakening protected-branch verification.

Proposed work

• Measure the current median and p95 time to first failure and final required-check completion.
• Map check dependencies and run independent lint, static-analysis, security, build, and test work concurrently.
• Split large test suites into deterministic CI shards where the suite size justifies it.
• Evaluate a conservative change-based early-feedback lane while retaining the complete suite as a required merge gate.
• Start static analysis as soon as its actual source and coverage inputs are ready rather than after unrelated verification stages.
• Combine coverage artifacts from shards when required by the quality gate.
• Improve dependency and build caching where it reduces repeated setup time without compromising reproducibility.

Acceptance criteria

• Existing required verification coverage remains mandatory before protected-branch integration.
• Independent checks no longer wait on unrelated stages.
• Any test sharding is deterministic and demonstrably executes every test exactly once per full run.
• Small, isolated changes receive materially earlier actionable feedback.
• Coverage and quality-gate inputs remain complete.
• CI documentation explains the fast-feedback path, full gate, shard ownership, and local failure-reproduction commands.
• Before/after median and p95 timings are recorded.