docs: extensions.blender.org listing kit (copy + headless promo rende…

[ugulay]

…rer)

• docs/extensions-listing.md: paste-ready release notes, listing description, per-preview-slot captions and form-field reference for the Blender Extensions upload.
• promo/render_listing_images.py: renders the icon (256, transparent), the 1920x1080 featured image and three 16:9 previews headless with the add-on's own core (shape outlines, robust booleans, vent_slats, radial_sets, panel-line sweeps, settle_chain cable over scene collision).
• Generated images land in promo/out/ (gitignored).

What does this PR do?

Type of change

• Bug fix
• New feature / tool
• Docs only
• Refactor / cleanup

Checklist

• New classes are added to the _classes tuple in __init__.py
• Logic lives in a pure core/ function; the operator stays thin
• core/ does not import bpy.ops / gpu / blf (boolean.py exception aside)
• Added/updated a test (tests/test_core.py for pure logic, tests/test_blender.py for bpy paths)
• python tests/test_core.py passes locally
• Updated the README key table if I added a user-facing action
• Checked the matching box in ROADMAP.md if applicable
• Code is PEP 8, ~90-char lines; comments/docstrings in English

Verified in Blender?

• Tested by hand in Blender 4.2+ (describe what you clicked through)
• Not applicable / pure-core only