fix: address CodeRabbit review on PR #393

- docs: scope the self.layout note to BasePlugin subclasses (others build
  a LayoutContext directly) and make explicit that adaptive layout is
  opt-in — classic rendering stays unless a plugin adopts the APIs.
- dev_server: broaden the render-request catch (a bad manifest.json now
  returns a clean 400 instead of an unhandled 500) and stop echoing raw
  exception text in the loader-failure responses — full tracebacks go to
  the dev server's console log instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FqzC1nzTWL4kaqgMaQZFam

docs/FONT_MANAGER.md

@@ -2,8 +2,10 @@
 
 > **Picking a size automatically:** if you want the *largest font that fits
 > a given area* rather than a fixed size, use the adaptive layout system's
-> font ladders (`self.layout.fit_text(...)`) which resolve through this
-> FontManager — see [ADAPTIVE_LAYOUT.md](ADAPTIVE_LAYOUT.md).
+> font ladders, which resolve through this FontManager. `BasePlugin`
+> subclasses get this as `self.layout.fit_text(...)`; other code can build
+> a `LayoutContext(width, height, font_manager)` directly — see
+> [ADAPTIVE_LAYOUT.md](ADAPTIVE_LAYOUT.md).
 
 ## Overview
 

docs/PLUGIN_DEVELOPMENT_GUIDE.md

@@ -2,10 +2,13 @@
 
 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.
+> **Rendering guidance:** plugins should read the display size dynamically
+> (`self.display_manager.matrix.width/height`) rather than hardcoding one
+> panel. For plugins that want to *scale* their layout to any panel, the
+> opt-in adaptive layout system ([ADAPTIVE_LAYOUT.md](ADAPTIVE_LAYOUT.md))
+> provides the shared helpers — fonts, images, and composite layouts that
+> scale. Existing plugins keep their classic rendering unless they adopt
+> those APIs; nothing migrates automatically.
 
 ## Overview