ci(codecov): Add release coverage with tag flags

[lens0021]

• Add release event trigger to rust workflow
• Skip build and test jobs on release events (tests already ran on the commit before tagging)
• Add rust flag to all codecov uploads for consistency
• Add release-coverage job that runs only on release events and uploads coverage with both rust and the release tag name (e.g., rust,1.0.0-alpha)
• Move LLVM_COV_VERSION to global env for better organization

Changes

1. Release event trigger: Workflow now triggers on release events with type released
2. Skip redundant jobs on release: Added if: github.event_name != 'release' to build, test, and macos-test jobs
3. Codecov flags:
   • Regular coverage job: uses rust flag only
   • Release coverage job: uses rust,{tag_name} flags (e.g., rust,1.0.0-alpha)
4. Environment optimization: Moved LLVM_COV_VERSION to workflow-level env

🤖 Generated with Claude Code

Test plan

• Verify workflow runs successfully on push/PR (existing behavior)
• Verify release-coverage job runs when a release is published