docs: add local coverage workflow

[rmorse]

Summary

• Add docs/COVERAGE.md with the local cargo-llvm-cov workflow.
• Document the Windows AV-safe target/temp directory setup used for coverage builds.
• Link coverage guidance from CONTRIBUTING.md and ignore common generated coverage reports.

Local coverage baselines from this branch

• cargo llvm-cov -p whisper-cpp-plus --json --summary-only -- --test-threads=1: 71.01% lines, 70.92% functions, 67.65% regions.
• cargo llvm-cov -p whisper-cpp-plus --features async --json --summary-only -- --test-threads=1: 66.99% lines, 62.16% functions, 62.31% regions.

Validation

• cargo llvm-cov -p whisper-cpp-plus --json --summary-only --output-path D:\cargo-coverage-whisper-cpp-plus-rs\coverage-whisper-cpp-plus.json -- --test-threads=1
• cargo llvm-cov -p whisper-cpp-plus --features async --json --summary-only --output-path D:\cargo-coverage-whisper-cpp-plus-rs\coverage-whisper-cpp-plus-async.json -- --test-threads=1
• cargo fmt --all -- --check
• git diff --check

Coverage remains local-only for now; CI integration can follow once we decide on reporting, badges, and whether to use Codecov/Coveralls or GitHub summaries.