Add HL7 v2 narrative extraction with source provenance

[maziyarpanahi]

Description

Adds de-identified narrative extraction for common HL7 v2 ADT, ORU, and ORM messages. The extractor reuses the existing segment-aware parser and structured redactor, renders stable flat or sectioned text, passes the complete narrative through the privacy pipeline, and preserves final-text offsets back to source segment occurrences and fields.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test addition/improvement

Changes Made

• Added extract_hl7v2_narrative with deterministic flat and sectioned modes for common message, patient, encounter, order, observation, result, and note fields.
• Added PHI-safe field provenance with final narrative offsets, segment indices and occurrences, field positions, section ranges, and lookup helpers.
• Added synthetic ADT/ORU coverage and a usage guide linked from the existing HL7 v2 documentation and docs navigation.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this change with different models/inputs

Validation completed:

• .venv/bin/python -m pytest tests/ -q — 5,205 passed, 46 skipped
• Focused HL7 v2 tests — 14 passed
• make format
• make lint
• make format-check
• make type-check
• make docs-build
• Scoped pre-commit hooks

Documentation

• I have updated the documentation accordingly
• I have added docstrings to new functions/classes
• I have updated the CHANGELOG.md

A changelog update is not needed for this focused roadmap feature PR.

Code Quality

• I ran make format, make lint, and make format-check
• For Swift/OpenMedKit changes, I ran make format-swift and make lint-swift
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

No Swift/OpenMedKit files are changed.

Dependencies

• I have not added any new dependencies
• OR I have added new dependencies and they are justified because: ____

Checklist

• I have read the contributing guidelines
• My commits have clear, descriptive messages
• I have squashed/organized my commits appropriately

Related Issues

Closes #861

Screenshots/Examples

Not applicable. This change adds a Python interoperability API, offset provenance contract, tests, and documentation without a UI surface.