ci(test): establish a ratcheting coverage floor

[joshrotenberg]

Summary

Claim issue #150 and turn the existing repository-wide coverage report into a modest, evidence-based regression gate.

Closes #150.

Baseline

The current all-features cargo llvm-cov baseline after #145 and #146 is:

• Lines: 2,976 / 3,897 = 76.366%
• Functions: 560 / 743 = 75.370%
• Regions: 2,912 / 3,887 = 74.916%

Changes

• Enforces an initial 76% repository-wide line floor with cargo llvm-cov.
• Separates report generation from the named coverage-regression step.
• Makes the coverage job blocking in the aggregate CI status.
• Makes Codecov upload failures explicit warnings while leaving the deterministic source gate authoritative.
• Documents local reproduction, the reviewed downward-change policy, and the initial decision not to enforce patch coverage.

Validation

• Full all-features coverage run: 2,976 / 3,897 lines = 76.366%; 76% gate passes.
• Deliberate 77% threshold against the same report exits 1, proving the regression gate.
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• Parsed .github/workflows/ci.yml as YAML.
• git diff --check