chore: prepare the 3.8.1 release (#756)

* chore: prepare the 3.8.1 release

Bumps src.__version__ to 3.8.1 and moves the Unreleased CHANGELOG entries
under a 3.8.1 heading, with an empty Unreleased above it.

The reason for the release is smooth scrolling at held-frame speeds. On
3.8.0 the default 50 px/s snapped to a stepped 48 px/s on a 120 Hz panel,
and any scroll slower than one pixel per refresh showed a half-pixel step
across the middle of the panel. Both are fixed on main (#710, #711) but
were in no release, so every stable-channel device still had them.

- #710's CHANGELOG entry had been filed under 3.8.0 although it merged
  after the v3.8.0 tag; it moves to 3.8.1.
- #711 had no CHANGELOG entry; it gets one.

scripts/check_release_version.py v3.8.1 passes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* docs: sports_game_over and draw_text_outlined ship in 3.8.1

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/__init__.py

@@ -4,5 +4,5 @@ LEDMatrix Display System
 Core source package for the LED Matrix Display project.
 """
 
-__version__ = "3.8.0"
+__version__ = "3.8.1"