fix: track unknowns through comprehensions

[rohan-patnaik]

What changed

• Let comprehensions iterate when only a child of the range is unknown.
• Mark matching range elements as unknown during iteration.
• Preserve the original attribute trail, e.g. items[2].field.

Why

Partial evaluation currently stops at the range variable for descendant unknowns. This lets expressions like items.exists(x, x.field == 'test') report the specific unknown child attribute.

Fixes #321.

Testing

• go test ./interpreter -run 'TestInterpreter_ComprehensionTracksUnknownAttribute|TestInterpreter$'
• go test ./interpreter