refactor(xcresult): name the places a test's file can come from

[dfrankland]

Stack 1/3 — no behaviour change.

The file we report for a failed test was assembled by an .or_else chain of Option<String>, where every link meant something different and none of them said so. fileName is the site an assertion fired, which for a helper is the helper's file; the call-stack fallback is whichever frame happened to be outermost. Reading the chain, they are indistinguishable.

This adds the vocabulary:

• FileSource — one variant per place we look, each documented with what it actually means
• FileCandidate — carries the source alongside the path, so a candidate can be traced back to where it came from instead of arriving as an anonymous string
• ReportedPath — normalizes once at construction. It was previously applied by hand at each call site, which is a silent bug the moment one is missed, or two paths are compared having been normalized differently.

Each source's extraction moves into a new file_attribution module as its own named function. The module doc states the thing the code could not: an .xcresult records where a failure was raised, never where a test is declared, so every source here is a proxy for the question we are actually asking — and the answer is what codeowners get resolved from.

xcresult_legacy.rs gets ~100 lines smaller.

Verification

No behaviour change: the sources are offered in the order the chain tried them and the first is taken. All 11 pre-existing integration tests pass unchanged, plus 10 new unit tests covering normalization, candidate ordering and provenance, the stack-frame filter, and document-location cleaning.

🤖 Generated with Claude Code

[trunk-io]

😎 This pull request was merged.

[trunk-staging-io]

   

Failed Test	Failure Summary	Logs
pending_quarantine_test should be quarantined when run with variant	A test marked as pending was expected to fail but unexpectedly passed.	Logs ↗︎
variant_quarantine_test should be quarantined when run with variant	A test expected the sum of 2 + 2 to be 5, but it was actually 4, indicating a failing assertion.	Logs ↗︎

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.28%. Comparing base (64052e5) to head (8579cd1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1158      +/-   ##
==========================================
+ Coverage   82.97%   83.28%   +0.31%     
==========================================
  Files          71       72       +1     
  Lines       16044    16110      +66     
==========================================
+ Hits        13312    13417     +105     
+ Misses       2732     2693      -39     

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[trunk-io]

   

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[trunk-io]

This pull request was merged into main as part of stacked PR 1162.