feat: run comparison - merged graph & change summary logic

[camielvs]

Description

Second PR in the Compare Runs stack. Builds two more pieces of logic on top of the diff engine — still no UI.

• Merged graph — takes the task/input/output diffs and turns them into a single combined graph that overlays both runs. Every task, input and output from either run becomes one node, and every connection is tagged as belonging to run A only, run B only, or both. This is what the Graph tab later renders.
• Change summaries — small helpers that turn a task or input/output diff into a short human caption (e.g. "component · 2 arguments · cache disabled", or "source rewired") for use as subdued labels on graph nodes.

Related Issue and Pull requests

Part of the Compare Runs stack. Builds on #2596; the next PR (#2598) 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

Pure logic, no standalone UI.

• Run the unit tests: npm run test -- buildMergedGraph summarizeChange
• Coverage includes unioning tasks from both runs into single nodes, tagging edges as A-only / B-only / shared, dropping edges to unknown tasks, wiring input→task→output edges, rewired outputs, and the wording of the change summaries.

To see it end-to-end, check out the top of the stack (#2603), enable the Compare runs flag, and open the Graph tab of a comparison.

Additional Comments

[github-actions]

🎩 Preview

A preview build has been created at: cmp-02-graph-summary-logic/5f5b90c

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

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