[hardening] Cross-paragraph annotation spans silently truncated

[MTCMarkFranco]

Enhancement

OpenXmlMarkupService.LocateParagraph returns the first paragraph that contains charStart and never considers charStart + span.Length. Spans that legitimately cross a paragraph boundary (e.g. a sentence rule that matches across a soft break, a heading + first body sentence) are silently truncated to the first paragraph.

Location: src/LambdaRag.Markup/OpenXmlMarkupService.cs LocateParagraph (~line 197)

Expected behaviour options (pick one in implementation):

1. Locate both start and end paragraphs; emit a single comment whose range crosses paragraphs (OOXML supports this — CommentRangeStart in P1, CommentRangeEnd in P2).
2. Reject cross-paragraph spans with a logged warning and a typed AnnotationApplicationFailure outcome rather than silent fallback.

Acceptance criteria

• Decision documented in the implementation PR.
• Unit test covering a cross-paragraph annotation: either correct multi-paragraph range OR explicit rejection with logged warning.
• Existing single-paragraph behaviour unchanged.
• Idempotency + engine genericity tests still pass.