Performance baseline pointers: add linux_latest and repoint CI

[iahncajigas]

Summary

Follow-up to PR #12 to improve Linux performance baseline maintainability and unblock stable mainline performance-parity runs.

Changes

• Added Linux baseline pointer files:
  • tests/performance/fixtures/python/performance_baseline_linux_latest.json
  • tests/performance/fixtures/python/performance_baseline_linux_latest.csv
• Refreshed dated Linux baseline (performance_baseline_linux_20260304.*) from the latest mainline workflow_dispatch run artifact (S/M/L), keeping dated provenance in-repo.
• Updated performance-parity CI workflow to compare against *_linux_latest.* pointer files.
• Updated docs/tests references to use the *_latest pointer while retaining dated fixture checks.
• Kept cycle checklist path references consistent as parity/CYCLE_VALIDATION_CHECKLIST.md.

Validation

• pytest -q passes locally.
• Local S/M/L benchmark harness run and comparator run executed successfully with local env-compatible previous baseline.