[SandObserver]

Summary

• Add visibleFieldFlags, which resolves each field's showIf by its position and returns one flag per field, and keep visibleFieldKeys as a wrapper over it.
• Resolve a showIf dependency against every field carrying the named key, so a duplicated controller key still gates its dependants.
• Apply visibility to the rendered rows by index in _wireShowIf, instead of looking each row up by field key.
• Cover a manifest whose two branches declare the same field key.

Testing
All green, including the homelab Docker build. Added 1 test for branch fields sharing a key, and updated the source-shape assertion that pinned the old lookup.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.