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/esp32/rom/esp32.rom.syscalls.ld

@@ -0,0 +1,66 @@
+/* These ROM functions call respective entries in the syscall table.
+   They are called by other ROM functions (mostly from newlib).
+   We don't link to them directly, since in IDF there are actual
+   implementations of these functions, with same names.
+
+   I.e.:
+
+   times (in ROM) -> _times_r (in ROM) -> syscall table entry _times_r -> _times_r (in IDF)
+
+   Hence the following entries are provided only for reference
+   and commented out.
+ */
+
+/*   <--- the following lines are commented out
+
+PROVIDE ( calloc = 0x4000bee4 );
+PROVIDE ( free = 0x4000beb8 );
+PROVIDE ( _free_r = 0x4000bbcc );
+PROVIDE ( _getpid_r = 0x4000bcfc );
+PROVIDE ( __getreent = 0x4000be8c );
+PROVIDE ( _gettimeofday_r = 0x4000bc58 );
+PROVIDE ( _kill_r = 0x4000bd10 );
+PROVIDE ( _lock_acquire = 0x4000be14 );
+PROVIDE ( _lock_acquire_recursive = 0x4000be28 );
+PROVIDE ( _lock_close = 0x4000bdec );
+PROVIDE ( _lock_close_recursive = 0x4000be00 );
+PROVIDE ( _lock_init = 0x4000bdc4 );
+PROVIDE ( _lock_init_recursive = 0x4000bdd8 );
+PROVIDE ( _lock_release = 0x4000be64 );
+PROVIDE ( _lock_release_recursive = 0x4000be78 );
+PROVIDE ( _lock_try_acquire = 0x4000be3c );
+PROVIDE ( _lock_try_acquire_recursive = 0x4000be50 );
+PROVIDE ( malloc = 0x4000bea0 );
+PROVIDE ( _malloc_r = 0x4000bbb4 );
+PROVIDE ( _raise_r = 0x4000bc70 );
+PROVIDE ( realloc = 0x4000becc );
+PROVIDE ( _realloc_r = 0x4000bbe0 );
+PROVIDE ( _sbrk_r = 0x4000bce4 );
+PROVIDE ( _system_r = 0x4000bc10 );
+PROVIDE ( _times_r = 0x4000bc40 );
+PROVIDE ( _close_r = 0x4000bd3c );
+PROVIDE ( _exit_r = 0x4000bd28 );
+PROVIDE ( _fstat_r = 0x4000bccc );
+PROVIDE ( _link_r = 0x4000bc9c );
+PROVIDE ( _lseek_r = 0x4000bd8c );
+PROVIDE ( _open_r = 0x4000bd54 );
+PROVIDE ( _read_r = 0x4000bda8 );
+PROVIDE ( _rename_r = 0x4000bc28 );
+PROVIDE ( _unlink_r = 0x4000bc84 );
+PROVIDE ( _write_r = 0x4000bd70 );
+
+    ---> end commented out block
+*/
+
+
+/* These are the non-reentrant versions of syscalls present in the ROM.
+   They call the reentrant versions, passing the pointer returned by __getreent
+   as the first argument.
+ */
+
+close = 0x40001778;
+open = 0x4000178c;
+read = 0x400017dc;
+sbrk = 0x400017f4;
+times = 0x40001808;
+write = 0x4000181c;