[hardening] DeletedRun writes literal '[deleted]' instead of original span text

[MTCMarkFranco]

Bug

OpenXmlMarkupService.ApplyOne writes a literal "[deleted]" token into the DeletedRun regardless of what was actually deleted.

Location: src/LambdaRag.Markup/OpenXmlMarkupService.cs line ~186

paragraph.AppendChild(new DeletedRun(new Run(new DeletedText("[deleted]") ...

Impact: When a reviewer accepts/rejects a tracked deletion in Word, they see the strikethrough text [deleted] instead of the actual original wording. This makes tracked-change accept/reject ambiguous and is unsuitable for legal review.

Expected: DeletedRun should contain the original span text (a.Span source slice) so accepting the deletion removes the right characters and rejecting it preserves them.

Acceptance criteria

• DeletedRun contains the actual original text from the span, not a placeholder.
• Unit test asserting that for a Replace annotation with original "foo bar" → "baz", the resulting docx contains a w:del with w:delText = "foo bar".
• Idempotency test still passes (deterministic output preserved).
• Engine genericity guard test still passes (no domain coupling introduced).

Out of scope

• Precise span anchoring (tracked separately).
• Cross-paragraph spans (tracked separately).