Add GGUF embedding-backbone export for grounding retrieval

[maziyarpanahi]

Description

Adds local F16 and Q8_0 GGUF export for embedding backbones used by the grounding retrieval path. The converter validates checkpoint configuration before starting, rejects token-classification heads with a descriptive error, writes both variants atomically, and emits publication-ready artifact metadata.

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 an embedding-only GGUF converter with F16 and Q8_0 outputs.
• Reject token-classification and other task-specific heads before conversion.
• Record canonical GGUF format plus per-artifact precision and quantization metadata.
• Document local setup, output layout, limitations, and manifest fields.

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:

• make format
• make lint
• make format-check
• make docs-build
• .venv/bin/python -m pytest tests/ -q (5,202 passed, 46 skipped)

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 #1463

Screenshots/Examples

The documented output contains model-f16.gguf, model-q8_0.gguf, config.json, and openmed-gguf.json.