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/memchr/.vim/coc-settings.json

@@ -0,0 +1,16 @@
+{
+  "rust-analyzer.cargo.allFeatures": false,
+  "rust-analyzer.linkedProjects": [
+    "benchmarks/engines/libc/Cargo.toml",
+    "benchmarks/engines/rust-bytecount/Cargo.toml",
+    "benchmarks/engines/rust-jetscii/Cargo.toml",
+    "benchmarks/engines/rust-memchr/Cargo.toml",
+    "benchmarks/engines/rust-memchrold/Cargo.toml",
+    "benchmarks/engines/rust-sliceslice/Cargo.toml",
+    "benchmarks/engines/rust-std/Cargo.toml",
+    "benchmarks/engines/stringzilla/Cargo.toml",
+    "benchmarks/shared/Cargo.toml",
+    "fuzz/Cargo.toml",
+    "Cargo.toml"
+  ]
+}