feat(plugins): adaptive-lib discoverability + advisory version compat warning

Discoverability: re-export the adaptive layout/image API from src.common
(the blessed-helpers package plugin authors already know) — canonical
paths stay src.adaptive_layout / src.adaptive_images so nothing breaks.
Document it in src/common/README.md and cross-link ADAPTIVE_LAYOUT.md
from the developer docs authors actually read (quick reference, API
reference, advanced dev, font manager, dev preview, plugin dev guide);
ADAPTIVE_LAYOUT.md gains adaptive-images, composite-layouts and
preserving-user-customization sections.

Compat: PluginLoader now logs one advisory warning (never raises) when a
plugin's manifest declares a min LEDMatrix version newer than the running
core, checking the min_ledmatrix_version / requires.* / versions[]
spellings found in the wild. Guarded against stale core version numbers.

src/__init__.py __version__ bumped 1.0.0 -> 3.1.0 to match the latest
release tag (v3.1.0) — it had never been updated and the compat check
needs a truthful number. NOTE: verify this matches the intended release
numbering before the next tag.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/PLUGIN_DEVELOPMENT_GUIDE.md

@@ -2,6 +2,11 @@
 
 This guide explains how to set up a development workflow for plugins that are maintained in separate Git repositories while still being able to test them within the LEDMatrix project.
 
+> **Rendering guidance:** plugins are expected to read the display size
+> dynamically and lay themselves out for any panel. The adaptive layout
+> system ([ADAPTIVE_LAYOUT.md](ADAPTIVE_LAYOUT.md)) provides the shared
+> helpers for that — fonts, images, and composite layouts that scale.
+
 ## Overview
 
 When developing plugins in separate repositories, you need a way to: