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/DEV_PREVIEW.md

@@ -6,6 +6,12 @@ Tools for rapid plugin development without deploying to the RPi.
 
 Interactive web UI for tweaking plugin configs and seeing the rendered display in real time.
 
+The size inputs have a preset dropdown with the harness's standard panel
+sizes, and the **All Sizes** button renders the current config at every
+harness size in a side-by-side gallery (`POST /api/render-matrix`) — the
+quickest way to eyeball adaptive-layout behavior across panels
+(see [ADAPTIVE_LAYOUT.md](ADAPTIVE_LAYOUT.md)).
+
 ### Quick Start
 
 ```bash