feat: support incremental diffs for GitHub merge queue

[danadajian]

Summary

GitHub's merge queue combines each queued PR with all preceding positions into a single commit, which causes visual regression checks to surface false positives — position N appears to have diffs from N-1's changes even when it didn't touch those components.

This PR introduces a merge-queue-diff workflow that eliminates that noise. It splits the merge queue check into two jobs: Job 1 runs visual tests and uploads screenshots as usual (skipping commit status and PR comments), and Job 2 waits for the preceding queue position to finish, then computes pixel-isolated diffs by comparing N's renders against N-1's renders rather than the shared base images. Job 2 sets the final commit status, so only N's own visual changes are reported. When N-1 had no diffs or failed, Job 2 falls back to base-image comparison directly.

The pr workflow is also hardened for merge queue runs: if Job 1 crashes, it now sets a failure commit status so Job 2 doesn't poll until timeout with no signal. Pixelmatch utilities are refactored from comparadise-utils into shared/ to support the new workflow.

How it works

Consumers add two jobs to their merge queue workflow:

on:
  merge_group:

jobs:
  visual-tests:        # Job 1: run tests, upload images, skip status/comment
    steps:
      - uses: ExpediaGroup/comparadise@main
        with:
          workflow: pr
          commit-hash: ${{ github.event.merge_group.head_sha }}

  visual-diff:         # Job 2: wait for N-1, compute isolated diffs, set status
    needs: visual-tests
    steps:
      - uses: ExpediaGroup/comparadise@main
        with:
          workflow: merge-queue-diff
          commit-hash: ${{ github.event.merge_group.head_sha }}

Job 2 polls N-1's commit status, then runs pixelmatch against N-1's uploaded renders. If N-1 had no diffs (success) or failed, it falls back to comparing against base-images/ directly.

Test plan

• Queue two PRs with overlapping visual changes — N should only show its own diffs, not N-1's
• Queue a PR with no visual changes behind one that has changes — N should show success
• Crash Job 1 during a merge queue run — Job 2 should see failure status and not poll until timeout
• Queue a PR at position 1 (no N-1) — should fall back to base-image comparison directly
• Accept diffs via the Comparadise UI using the commit status target URL — status should flip to success

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