Add entity linking confidence support

[Travmatth]

Summary

• add opt-in entity-linking score and limit parameters to redact() and group_entities()
• parse filtered entities, indexed groups, and the symmetric confidence matrix returned by TonicAI/solar#3483
• add iter_entity_links(min_confidence=..., confidence_type=...) so callers can process each pair once without handling matrix symmetry
• retain parsing support for the prior embedded-entity grouping response
• document confidence filtering with an SDK example

Validation

• ruff check --target-version py39 on changed Python files
• 32 hermetic tests passed
• live smoke test against Solar PR app #3483 parsed the 8x8 customer-reproduction matrix and excluded the 0.895 false link at a 0.90 threshold

Depends on TonicAI/solar#3483.