refactor: run slang semantic diagnostics asynchronously

[hongjr03]

Summary

• Move whole-profile slang semantic diagnostics into a background semantic compiler actor.
• Keep document and workspace diagnostic request paths syntax-only, then surface semantic diagnostics through the external diagnostic cache.
• Preserve pull/push diagnostic refresh behavior, result IDs, and quick fixes that depend on cached semantic diagnostics.
• Update diagnostics tests to wait for asynchronous semantic results.

Validation

• cargo fmt --all
• git diff --check
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings

[github-actions]

Docs preview: https://vide.pascal-lab.net/preview/pr-287/