build(coverage): switch from grcov to cargo-llvm-cov

[robdefeo]

Summary

The project used grcov for coverage with a 60% threshold and inline grcov-excl-* annotations to exclude certain blocks. grcov requires manual RUSTFLAGS plumbing and a separate profraw collection step. This migrates to cargo-llvm-cov (simpler invocation, no manual env vars), raises the threshold to 80%, and removes the now-dead exclusion annotations. Four new tests are added to cover the previously untested downmix paths that the annotation removal exposed.

Coverage after: 86.7% (up from 75.8% under the old tool).

Changes

• Replace grcov with cargo-llvm-cov in mise.toml and justfile
• Raise coverage threshold 60% → 80%; exclude src/main.rs (binary entry point, not unit-testable) from the report denominator
• Remove grcov-excl-start / grcov-excl-stop annotations from src/main.rs, src/output.rs, src/transcribe.rs, src/audio.rs
• Add downmixes_7_1_to_mono — covers SIDE_LEFT | SIDE_RIGHT arm
• Add downmixes_rear_centre_to_mono — covers REAR_CENTRE arm
• Add downmixes_all_zero_weight_channels_uses_arithmetic_mean — covers the BS.775 fallback arithmetic mean path
• Add bs775_weight_returns_zero_for_unrecognized_channel — covers the _ => 0.0 wildcard arm

[greptile-apps]

Greptile Summary

This PR migrates code coverage from grcov to cargo-llvm-cov, raises the threshold from 60% to 80%, excludes src/main.rs from the report denominator, and removes the now-dead grcov-excl-* annotations. Four new unit tests cover the previously excluded downmix_to_mono paths (7.1 surround, rear-centre, all-zero-weight arithmetic mean fallback, and the bs775_weight wildcard arm), and the math in each test is correct.

Confidence Score: 5/5

Safe to merge — clean tooling migration with correct new tests and no functional regressions.

All changes are in the build/CI tooling and test additions. The new four tests are mathematically correct (verified against bs775_weight coefficients and the arithmetic-mean fallback logic). The cargo-llvm-cov two-step workflow (--no-report → report) is the canonical pattern for generating multiple report formats. No P0 or P1 findings.

No files require special attention.

Important Files Changed

Filename	Overview
justfile	Replaces grcov invocation with cargo-llvm-cov two-step workflow (--no-report then report), raises threshold to 80%, and removes manual RUSTFLAGS/LLVM_PROFILE_FILE plumbing — all correct.
mise.toml	Swaps cargo:grcov 0.10.7 for cargo:cargo-llvm-cov 0.8.5; straightforward dependency swap.
src/audio.rs	Adds four new unit tests (7.1 downmix, rear-centre downmix, all-zero-weight arithmetic mean, bs775_weight wildcard); all test assertions are mathematically correct.
src/main.rs	Removes grcov-excl-start/stop annotations around the binary entry point; no functional change.
src/output.rs	Removes grcov-excl-start/stop around the test module; no functional change.
src/transcribe.rs	Removes grcov-excl-start/stop from the transcribe function and test module; no functional change.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[just coverage] --> B[mkdir -p coverage]
    B --> C[cargo llvm-cov --no-report\nruns tests + collects profdata]
    C --> D[cargo llvm-cov report --lcov\n--ignore-filename-regex src/main.rs\n→ coverage/lcov]
    C --> E[cargo llvm-cov report --html\n--ignore-filename-regex src/main.rs\n→ coverage/]
    D --> F[lcov --summary coverage/lcov\nextract line %]
    F --> G{line % >= 80?}
    G -->|yes| H[✅ pass]
    G -->|no| I[❌ exit 1]

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