Add ISCII and legacy-font Devanagari conversion

[maziyarpanahi]

Description

Adds deterministic, local ISCII-1991 and caller-supplied legacy-font conversion before Indic normalization and PII routing. The conversion preserves source-byte alignment so spans detected on Unicode text can be reported against the original legacy stream, while conservative heuristics leave ordinary Latin and valid UTF-8 input unchanged.

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

• Add bidirectional ISCII-1991 Devanagari conversion with contextual nukta, virama, join-control, extension, NFC, and byte-offset handling.
• Add conservative ISCII and suspicious ASCII-cluster detection plus immutable JSON/YAML legacy-font maps without bundled proprietary font data.
• Integrate conversion into detection and pipeline normalization, preserve attached Indic marks, and expose PHI-safe conversion metadata.
• Document usage, provenance, licensing, offset mapping, and the Vedic/INV limitations.

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:

• .venv/bin/python -m pytest tests/ -q — 5215 passed, 46 skipped
• make format
• make lint
• make format-check
• make type-check
• make docs-build

Documentation

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

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

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 #1486

Screenshots/Examples

cf cc e1 d5 20 d5 cf e8 cc da converts to रमेश शर्मा and encodes back to the identical ISCII bytes.