chore(scroll): drop the dead sub-pixel path, and two dev-tooling papercuts (#570)

Three independent changes, none of which alter runtime behaviour.

1. Remove ScrollHelper._get_visible_portion_subpixel and
   _interpolate_subpixel (162 lines). get_visible_portion dispatches only to
   _blend_visible_portion, so _get_visible_portion_subpixel had no caller, and
   _interpolate_subpixel was reachable only from inside it -- a closed island.
   _blend_visible_portion's own docstring already records that the scipy path
   it replaced was dead; the replacement landed but the corpse stayed.

2. scripts/check_plugin.py: also search ../ledmatrix-plugins/plugins. The
   scoreboards live in the sibling checkout, so --all silently skipped every
   one of them and only --plugin-dir reached them.

3. .gitignore: ignore config/.config_secrets.json.tmp.*, which the suite
   leaves behind several of per run.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -5,6 +5,9 @@ __pycache__/
 
 # Secrets
 config/config_secrets.json
+# Atomic writes leave these behind when a save or a test is interrupted;
+# the suite drops several per run.
+config/.config_secrets.json.tmp.*
 config/config.json
 config/config.json.backup
 config/wifi_config.json