Allow 0.1% codecov project coverage threshold
Summary
codecov/project uses a zero threshold, so any drop fails it. Coverage fluctuates between runs even when no source changes: across the last 12 dependency-only commits on main it moved between -0.05% and +0.03%, and the check failed on 6 of them. #150 hit the same thing (-0.01%, from StorageCommitmentRequest.cs and Dwt97.cs, neither touched by the PR).
Adds .github/codecov.yml allowing a 0.1% project drop, which still catches real regressions. The patch check is unchanged.
The underlying run-to-run variation (hits dropping in deterministic tests such as StorageCommitmentTests) is not addressed here.
Summary by cubic
Adds a 0.1% coverage threshold for codecov's project check so small run-to-run variations no longer fail it.
Coverage fluctuates up to ±0.05% between identical runs, failing the check on about half of recent main commits even with no source changes. The new threshold still catches real regressions, and the patch check is unchanged.