attributed_text: add reusable sweep segmentation for unchecked spans

[waywardmonkeys]

The main public addition is AttributeSegmentsWorkspace::for_each_span_segment_uncheattributed_text: add reusable sweep segmentation for unchecked spans

This adds a reusable sweep-line path to attributed_text so callers can segment a trusted span slice without rebuilding a temporary AttributedText.

The main public addition is AttributeSegmentsWorkspace::for_each_span_segment_unchecked, which reports each resolved segment plus the active span indices in application order. The name is intentionally explicit: the method does not validate in release builds that the TextRange values belong to the text length supplied by the caller.

This also adds AttributedText::from_attributes_unchecked for constructing attributed text from an existing attribute vector while preserving application order. Callers should prefer apply_attribute_bytes or TextRange::new unless the spans have already been checked against the exact text storage.

This is needed by the styled-text builder path, which already has patch ranges in TextRange form and wants to resolve overlaps without allocating an intermediate attributed-text value. The API is callback-based so the sweep result can be exposed without committing to extra iterator structs.

[tomcur]

This looks good.

My main concern is somewhat orthogonal and take-it-or-leave-it: the API uses the word "validated" for parts where the data is assumed to be correct, whereas often in Rust that relation is flipped, using try_, checked_, or some other term for APIs that validate (otherwise assuming the data passed in is correct).

[waywardmonkeys]

This assumes the caller validated it. More of an unchecked.