feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/basic-toml/tests/valid/comments-everywhere.toml

@@ -0,0 +1,24 @@
+# Top comment.
+  # Top comment.
+# Top comment.
+
+# [no-extraneous-groups-please]
+
+[group] # Comment
+answer = 42 # Comment
+# no-extraneous-keys-please = 999
+# In between comment.
+more = [ # Comment
+  # What about multiple # comments?
+  # Can you handle it?
+  #
+          # Evil.
+# Evil.
+  42, 42, # Comments within arrays are fun.
+  # What about multiple # comments?
+  # Can you handle it?
+  #
+          # Evil.
+# Evil.
+# ] Did I fool you?
+] # Hopefully not.