codeSplitter shadow mode: run our merge beside git's on real PRs, file every disagreement #1014

Follow-up to #938: catch our own merge mistakes.

Run codeSplitter's line-id merge next to git merge-tree on real PRs (this repo's merged PRs make a good corpus: base, head and main at merge time). Record every file where:

• git conflicts and codeSplitter merges clean (check that the result builds and passes the tests),
• codeSplitter conflicts and git merges clean (a resolver gap, like the ones measured in docs/merge-solutions.md),
• the two clean results differ,
• a guard trips (markers, unparseable data, lint).

Each disagreement becomes a fixture in test/conflicts.test.ts or a fix issue. Start as a script over local git history, with no live resources.

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