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/riscv/src/bits.rs

@@ -0,0 +1,17 @@
+/// Insert a new value into a bitfield
+///
+/// `value` is masked to `width` bits and inserted into `orig`.`
+#[inline]
+pub fn bf_insert(orig: usize, bit: usize, width: usize, value: usize) -> usize {
+    let mask = (1 << width) - 1;
+    orig & !(mask << bit) | ((value & mask) << bit)
+}
+
+/// Extract a value from a bitfield
+///
+/// Extracts `width` bits from bit offset `bit` and returns it shifted to bit 0.s
+#[inline]
+pub fn bf_extract(orig: usize, bit: usize, width: usize) -> usize {
+    let mask = (1 << width) - 1;
+    (orig >> bit) & mask
+}