ci: add Scalpel 0.3.0 shadow comparison alongside grep-based detection

[gnodet]

Summary

Upgrade Maveniverse Scalpel from extension3:0.1.0 to extension:0.3.0 and run it in shadow mode — observing what skip-tests mode would do without affecting actual test execution.

This allows validating Scalpel's decisions across many PRs before switching to Scalpel-driven test execution in a follow-up PR.

Changes

• Scalpel 0.3.0: Upgrade with two new features:
  • Source-set-aware propagation (test-jar vs regular dependents)
  • skipTestsForDownstreamModules (mirrors EXCLUSION_LIST)
• CI workflow: Pre-fetch base branch for Scalpel merge-base detection, remove persist-credentials: false (needed for git fetch)
• Shadow comparison: Add collapsible section in PR comments showing what Scalpel skip-tests mode would have tested
• Documentation: Update CI-ARCHITECTURE.md with shadow comparison details

How it works

1. Existing grep-based + Scalpel report detection continues to drive actual test execution (no change)
2. Scalpel 0.3.0 additionally computes what skip-tests mode would do (with skipTestsForDownstreamModules matching the current EXCLUSION_LIST)
3. The PR comment gets a collapsible "Scalpel 0.3.0 shadow comparison" section showing:
   • How many modules Scalpel would test (direct + downstream)
   • Which downstream modules would have tests skipped
   • Full module lists for review

Risk

Zero — Scalpel runs in report mode only. Test execution is completely unchanged. If Scalpel fails, the script falls back gracefully with no impact.

Claude Code on behalf of Guillaume Nodet

[github-actions]

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