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/proc-macro-error/tests/ui/emit.stderr

@@ -0,0 +1,48 @@
+error: emit!(span, from) test
+ --> $DIR/emit.rs:4:7
+  |
+4 | emit!(one, two, three, four, five);
+  |       ^^^
+
+error: emit!(span, expr1, expr2) test
+ --> $DIR/emit.rs:4:12
+  |
+4 | emit!(one, two, three, four, five);
+  |            ^^^
+
+error: emit!(span, single_expr) test
+ --> $DIR/emit.rs:4:17
+  |
+4 | emit!(one, two, three, four, five);
+  |                 ^^^^^
+
+error: Diagnostic::emit() test
+ --> $DIR/emit.rs:4:24
+  |
+4 | emit!(one, two, three, four, five);
+  |                        ^^^^
+
+error: emit_call_site_error!(expr) test
+ --> $DIR/emit.rs:4:1
+  |
+4 | emit!(one, two, three, four, five);
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: This is an error
+
+  = note: simple note
+  = help: simple help
+  = help: simple hint
+  = note: simple yay
+  = note: format note
+  = note: Some note
+  = note: spanned simple note
+  = note: spanned format note
+  = note: Some note
+
+ --> $DIR/emit.rs:5:13
+  |
+5 | emit_notes!(one, two);
+  |             ^^^