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/split_at_dynamic_padding.x86-64

@@ -0,0 +1,12 @@
+bench_split_at_dynamic_padding:
+	mov rax, rdi
+	cmp rcx, rdx
+	jbe .LBB5_2
+	xor esi, esi
+	mov qword ptr [rax], rsi
+	ret
+.LBB5_2:
+	mov qword ptr [rax + 8], rdx
+	mov qword ptr [rax + 16], rcx
+	mov qword ptr [rax], rsi
+	ret