Document local v2 coverage report generation

[tkilias]

Goal

Document how contributors can generate and inspect v2 coverage reports locally.

Scope

• Run Bazel coverage with fuzz targets excluded.
• Print LCOV summary and file-by-file text reports.
• Generate an HTML report with genhtml.
• Generate the CI-compatible SonarQube coverage report.

Acceptance criteria

A contributor can follow the developer guide to generate and inspect v2 coverage locally.