feat: run comparison - shared diff UI primitives

[camielvs]

Description

Third PR in the Compare Runs stack. Adds the small, shared UI building blocks that the later comparison views reuse, so they stay consistent. Nothing here is wired into a page yet.

• CompareRunPicker — a list of recent runs used to pick a run for either side of the comparison.
• DiffStatusBadge — the coloured Added / Removed / Changed / Unchanged badge.
• RunTag / RunTags — the small "A" / "B" pills that mark which run a value belongs to.
• FieldDiffRow — a single field's before/after values with expand-to-see-more for long values.
• YamlDiffView — a side-by-side YAML diff of the two run specs (Monaco diff editor).

Related Issue and Pull requests

Part of the Compare Runs stack. Builds on #2597; the next PR (#2599) builds on this branch.

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

These are shared presentational components with no entry point of their own on this branch. They're exercised through the full feature.

• Check out the top of the stack (feat: run comparison - compare page, run selection & routing #2603) and enable the Compare runs beta flag (Settings → Beta features).
• Compare two runs and confirm the status badges, A/B run tags and field rows render correctly, and that the YAML tab shows a readable side-by-side diff.

Additional Comments

[github-actions]

🎩 Preview

A preview build has been created at: cmp-03-diff-ui-primitives/8a70f0f

[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
• feat: run comparison - run picker & switcher #2634
• 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 👈 (View in Graphite)
• feat: run comparison - merged graph & change summary logic #2597
• feat: run comparison - pipeline diff engine #2596
• master

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