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/zerocopy/benches/read_from_prefix.x86-64

@@ -0,0 +1,14 @@
+bench_read_from_prefix_static_size:
+	cmp rsi, 5
+	jbe .LBB5_2
+	mov eax, dword ptr [rdi]
+	movzx edi, word ptr [rdi + 4]
+	shl rdi, 32
+	or rdi, rax
+.LBB5_2:
+	shl rdi, 16
+	inc rdi
+	xor eax, eax
+	cmp rsi, 6
+	cmovae rax, rdi
+	ret