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/invalid/table-array-implicit.toml

@@ -0,0 +1,14 @@
+# This test is a bit tricky. It should fail because the first use of
+# `[[albums.songs]]` without first declaring `albums` implies that `albums`
+# must be a table. The alternative would be quite weird. Namely, it wouldn't
+# comply with the TOML spec: "Each double-bracketed sub-table will belong to 
+# the most *recently* defined table element *above* it."
+#
+# This is in contrast to the *valid* test, table-array-implicit where
+# `[[albums.songs]]` works by itself, so long as `[[albums]]` isn't declared
+# later. (Although, `[albums]` could be.)
+[[albums.songs]]
+name = "Glory Days"
+
+[[albums]]
+name = "Born in the USA"