ci(coverage): scope Test Coverage concurrency by ref (stop cross-PR cancellations)

[breimanntools]

Problem

The Test Coverage workflow (test_coverage.yml) sets its concurrency group to ${{ github.workflow }} — the workflow name only, with no ${{ github.ref }}. That makes the group repo-global: with cancel-in-progress: true, a new run on any branch/PR cancels the in-progress runs of all others.

With several PRs open at once, only the newest run survives; every other PR's coverage run ends as a spurious cancelled (red ✗) and never reports a coverage number. (Confirmed: the failing checks conclude cancelled, killed during dependency install before any test ran — not a test or coverage failure.)

Fix

Add ${{ github.ref }} to the group so it is scoped per branch/PR:

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true

Now a new push to a branch only supersedes that branch's own in-progress run (still no stacked redundant runs on rapid pushes), while different branches/PRs no longer cancel each other — so every PR gets its own coverage result. Standard GitHub Actions pattern. The explanatory comment is updated to match.

Trade-off: with many active branches this permits more than one coverage run at a time (the previous config deliberately serialized the heaviest workflow repo-wide). That serialization is what caused the missing coverage on all-but-one PR, so per-ref scoping is the correct behavior for a multi-PR repo.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.93%. Comparing base (1a152de) to head (cecafac).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #333   +/-   ##
=======================================
  Coverage   94.93%   94.93%           
=======================================
  Files         185      185           
  Lines       17883    17883           
  Branches     3038     3038           
=======================================
  Hits        16978    16978           
  Misses        598      598           
  Partials      307      307           

Components	Coverage Δ
cpp_core	94.95% <ø> (ø)

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