feat: run comparison - compare page, run selection & routing

[camielvs]

Description

Final PR in the Compare Runs stack — this is the one that turns all the previous pieces into a usable feature.

• Adds the Compare page (/compare) that ties everything together: a header with both runs (labelled A/B, swap, share-link and close), the run metadata bar, and the Structured / YAML / Graph tabs.
• Adds the ways to get there:
  • Dashboard run list — checkboxes on each row; selecting exactly two runs shows a "Compare" action bar.
  • Run view menu — a "Compare with another run…" item that pre-fills that run as side A.
• If you land on /compare without both runs chosen, it shows a run picker for the missing side (and guards against comparing a run with itself).
• Everything is gated behind a new compare-runs beta flag — the route redirects away and the entry points stay hidden when it's off.

Also moves the shared buildTaskExecutionStatusMap helper into utils/executionStatus (used by both the run view and the comparison).

Related Issue and Pull requests

Final PR in the Compare Runs stack. Builds on #2602 (#2596 → #2603).

Type of Change

• New feature

Checklist

• I have tested this does not break current pipelines / runs functionality
• I have tested the changes on staging

Screenshots (if applicable)

Test Instructions

1. Enable the Compare runs flag under Settings → Beta features.
2. From the dashboard: select exactly two runs with the row checkboxes — a Compare bar appears at the bottom. Click Compare.
3. From a run: open a run's menu and choose Compare with another run…, then pick a second run.
4. On the compare page, confirm:
   • The header shows both runs (A/B); swap flips them, share copies the URL, close returns to the run list.
   • The Structured, YAML and Graph tabs all work.
   • Opening the page URL directly (with ?a=…&b=…) loads the same comparison; picking the same run twice is blocked.
5. Flag off: confirm the checkboxes, compare bar and run-menu item disappear, and visiting /compare redirects to the run list.
6. Regression: open a normal run view and confirm task statuses still render (the buildTaskExecutionStatusMap move).

Additional Comments

[github-actions]

🎩 Preview

A preview build has been created at: cmp-08-compare-page-and-wiring/89ec2e4

[camielvs]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: v2 - always-on Compare page with in-page run switching #2604
• feat: run comparison - compare page, run selection & routing #2603 👈 (View in Graphite)
• feat: run comparison - graph diff view #2602
• feat: run comparison - structured diff view #2601
• feat: run comparison - run metadata comparison #2600
• feat: run comparison - artifact comparison #2599
• feat: run comparison - shared diff UI primitives #2598
• feat: run comparison - merged graph & change summary logic #2597
• feat: run comparison - pipeline diff engine #2596
• master

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