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/esp-hal/ld/esp32c3/memory.x

@@ -0,0 +1,34 @@
+MEMORY
+{
+    /*
+        https://github.com/espressif/esptool/blob/ed64d20b051d05f3f522bacc6a786098b562d4b8/esptool/targets/esp32c3.py#L78-L90
+        MEMORY_MAP = [[0x00000000, 0x00010000, "PADDING"],
+                  [0x3C000000, 0x3C800000, "DROM"],
+                  [0x3FC80000, 0x3FCE0000, "DRAM"],
+                  [0x3FC88000, 0x3FD00000, "BYTE_ACCESSIBLE"],
+                  [0x3FF00000, 0x3FF20000, "DROM_MASK"],
+                  [0x40000000, 0x40060000, "IROM_MASK"],
+                  [0x42000000, 0x42800000, "IROM"],
+                  [0x4037C000, 0x403E0000, "IRAM"],
+                  [0x50000000, 0x50002000, "RTC_IRAM"],
+                  [0x50000000, 0x50002000, "RTC_DRAM"],
+                  [0x600FE000, 0x60100000, "MEM_INTERNAL2"]]
+
+    ICACHE : ORIGIN = 0x4037C000,  LENGTH = 0x4000
+    /* Instruction RAM */
+    IRAM : ORIGIN = 0x4037C000 + 0x4000, LENGTH = 313K - 0x4000
+    /* Data RAM */
+    DRAM : ORIGIN = 0x3FC80000, LENGTH = 313K
+    
+    /* memory available after the 2nd stage bootloader is finished */
+    dram2_seg ( RW )       : ORIGIN = ORIGIN(DRAM) + LENGTH(DRAM), len = 0x3fcde710 - (ORIGIN(DRAM) + LENGTH(DRAM))
+
+    /* External flash */
+    /* Instruction ROM */
+    IROM : ORIGIN =   0x42000000 + 0x20, LENGTH = 0x400000 - 0x20
+    /* Data ROM */
+    DROM : ORIGIN = 0x3C000000, LENGTH = 0x400000
+
+    /* RTC fast memory (executable). Persists over deep sleep. */
+    RTC_FAST : ORIGIN = 0x50000000, LENGTH = 0x2000 /*- ESP_BOOTLOADER_RESERVE_RTC*/    
+}