Add Codecov coverage reporting (Rust)

[rmems]

Summary

Add Codecov integration for Rust code coverage reporting on the cortex-tensor crate.

Currently CI runs cargo test (in .github/workflows/rust.yml) but produces no coverage data or reports.

See: https://about.codecov.io/language/rust/

Motivation

• Track and improve test coverage over time
• Surface untested paths in critical areas (MoE routing, GGUF parsing, dequantization, tensor ops, transformer blocks)
• Get PR comments / status checks from Codecov
• Align with org practices (other Limen-Neural / rmems repos have used Codecov)

Recommended approach (modern Rust + Codecov)

Preferred tool: cargo-llvm-cov (excellent support for recent Rust editions, including 2024; produces clean LCOV).

Example steps in CI:

- name: Install cargo-llvm-cov
  uses: taiki-e/install-action@cargo-llvm-cov

- name: Generate coverage
  run: cargo llvm-cov --all-features --workspace --lcov --output-path lcov.info

- name: Upload to Codecov
  uses: codecov/codecov-action@v4
  with:
    files: lcov.info
    fail_ci_if_error: true
    token: ${{ secrets.CODECOV_TOKEN }}

Handle missing token gracefully (like the existing Snyk steps) so forks/PRs from external contributors don't break.

Scope / Tasks

• Add coverage collection step to the Rust workflow (or a matrix job)
• Add codecov.yml at repo root with Rust-appropriate settings:
  • ignore: ["target/", "examples/"]
  • comment: on PRs
  • coverage thresholds if desired (start lenient)
• Add Codecov badge to README.md (shields or codecov.io link)
• Document the token requirement in any CI docs / workflow comments
• Ensure cargo test behavior is unchanged when coverage is not collected
• (Optional) Wire into Azure Pipelines if we want cross-platform coverage numbers

Non-goals

• Do not change the public API or add runtime dependencies for coverage
• Do not enforce strict coverage gates on day one (can be added later)

Acceptance Criteria

• cargo llvm-cov (or equivalent) runs successfully in CI and produces lcov.info
• Coverage is uploaded to Codecov for pushes to main and for PRs
• Badge visible in README pointing to https://codecov.io/gh/Limen-Neural/cortex-tensor
• codecov.yml present and committed
• CI jobs still succeed on forks / when CODECOV_TOKEN is not set (use continue-on-error or conditional like Snyk)
• New runs appear on https://codecov.io/gh/Limen-Neural/cortex-tensor

References

• https://about.codecov.io/language/rust/
• https://github.com/codecov/codecov-action
• https://github.com/taiki-e/cargo-llvm-cov (recommended for Rust)
• Current workflow: .github/workflows/rust.yml

Suggested branch: feat/add-codecov

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Created via agent request for structured coverage addition.

[linear-code]

LIM-888