Three-part fix for enrich_hf_metadata failing to extract correct preview_url
from HuggingFace collection repos where models share flat heading levels:
1. _strip_standalone_images() now converts <img> tags to markdown image
syntax  instead of stripping the URL entirely, so the LLM
can still extract preview URLs.
2. _extract_section() uses a line-count-based forward window (stopping at
<a id> anchors) for non-heading matches, instead of stopping at the
very next heading. This prevents same-level sub-headings (# Download,
# Trigger, # Sample prompt within a single model section) from
truncating the window before sample images are included.
3. Post-processor preview fallback now filters gallery images to the
model-specific README section before falling back to the repo-wide
first image.
7b19bbb14e