build, add code coverage

[steven-varga]

[steven-varga]

Tracking

Implementation is in flight as PR #11 (branch `6-codecov-coverage`), which adds the lcov + Codecov v5 plumbing.

Sequencing

Per the workspace plan, PR #11 should land after #12 (CI alignment) and #13 (fixture pack) — otherwise the coverage metric reports against a dormant CI matrix and an empty test surface.

Action items on PR #11 (already commented there):

1. Retarget base branch from `release` to `staging` (new workspace canon).
2. Rebase onto staging after ci, align CI matrix with h5cpp staging and replace HDF5 dep with LLVM toolchain #12 and test, add transform fixture pack for coverage #13 merge.
3. First coverage number after the full chain becomes the baseline against test, add transform fixture pack for coverage #13's 94% target.

This issue closes when PR #11 merges.