Add hosted PR performance calibration workflow

[justinjoy]

Summary

Add a manually dispatched, hosted Ubuntu calibration campaign for an open PR. It freezes the PR base, head, and merge SHAs, then records six sequential runner observations. Each observation builds and measures two immutable revisions, with balanced base/merge order and same-revision controls.

The collector preserves raw CRDT and CSPA samples, correctness, CoV, target verdicts, build and host metadata, logs, and partial artifacts. It classifies target misses separately from skips and failures; incomplete or ineligible campaigns exit unsuccessfully.

This supplies the evidence needed for #2085 and the failing Perf Suite on #2077. It does not change the required PR performance check or its targets. The workflow uses GitHub-hosted ubuntu-latest, not the dedicated perf runner.

Validation

• 17 focused calibration tests passed in a uv virtual environment.
• actionlint .github/workflows/perf-hosted-calibration.yml passed.
• Python compilation and git diff --check passed.
• Independent Reviewer, Architect, and Critic approved commit 89422921457b820ad9244c44ef06389fd3b0f9b6.

Next step

After this workflow is available on main, dispatch it with PR number 2077. Review eligible paired evidence before changing the required performance gate or #2077 source. A skipped or incomplete campaign does not count as a timing result.

Refs #2085; diagnostic for #2077.