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/ADVANCED_PLUGIN_DEVELOPMENT.md

@@ -2,6 +2,12 @@
 
 Advanced patterns, examples, and best practices for developing LEDMatrix plugins.
 
+> **Adaptive layout:** for plugins that should render legibly on any panel
+> size (fonts that grow on big panels, layouts that degrade gracefully on
+> small ones), use the adaptive layout system — `self.layout`, `draw_fit`,
+> `draw_image`, `scoreboard_regions` — documented in
+> [ADAPTIVE_LAYOUT.md](ADAPTIVE_LAYOUT.md).
+
 ## Table of Contents
 
 - [Using Weather Icons](#using-weather-icons)