CI: target Swift CodeQL builds

[MarcusJRLee]

Summary

• Replace failing Swift autobuild with explicit macOS and iOS build steps.
• Run the extended Swift query suite only for analyzable source/build changes.
• Retain an immediate Analyze (swift) gate for unrelated PRs.
• Document the required gate behavior.

Verification

• scripts/check.sh
• Exact manual macOS+iOS workflow build locally.
• Workflow YAML parse and git diff --check.

Boundaries

• No product, signing, version, or release behavior changes.
• Action dependencies are immutable commit pins.
• Security-event permission is scoped to this workflow.