fix: preserve annotations across type boundaries and duplicate positions

[masonkatz]

Annotations store absolute row indexes, but text rendering splits rows into type-specific table groups whose indexes restart at zero. As a result, annotations placed at a type boundary can disappear. Additionally, flush consumes at most one annotation at a row index, so consecutive Annotate calls at the same position discard all but the first annotation.

Acceptance criteria

1. Add regression tests for annotations immediately before a struct type change and for multiple annotations at the same row position.
2. Verify each annotation is emitted in insertion order at its intended location.
3. Fix annotation tracking/rendering so grouping by struct type cannot invalidate annotation positions.
4. Keep annotations intentionally excluded from JSON and YAML output.
5. Run the relevant test suite.