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-derive/tests/ui/privacy.msrv.stderr

@@ -0,0 +1,147 @@
+error[E0747]: type provided when a constant was expected
+   --> $DIR/privacy.rs:105:9
+    |
+105 |         _,
+    |         ^
+    |
+    = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+   --> $DIR/privacy.rs:112:9
+    |
+112 |         _,
+    |         ^
+    |
+    = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+   --> $DIR/privacy.rs:121:9
+    |
+121 |         _,
+    |         ^
+    |
+    = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+   --> $DIR/privacy.rs:128:9
+    |
+128 |         _,
+    |         ^
+    |
+    = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+   --> $DIR/privacy.rs:135:52
+    |
+135 |     let _: <Union as zerocopy_renamed::HasField<_, _, { zerocopy_renamed::ident_id!(a) }>>::Type =
+    |                                                    ^
+    |
+    = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+   --> $DIR/privacy.rs:138:52
+    |
+138 |     let _: <Union as zerocopy_renamed::HasField<_, _, { zerocopy_renamed::ident_id!(b) }>>::Type =
+    |                                                    ^
+    |
+    = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+   --> $DIR/privacy.rs:145:51
+    |
+145 |     let _: <Enum as zerocopy_renamed::HasField<_, _, { zerocopy_renamed::ident_id!(0) }>>::Type =
+    |                                                   ^
+    |
+    = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+   --> $DIR/privacy.rs:148:51
+    |
+148 |     let _: <Enum as zerocopy_renamed::HasField<_, _, { zerocopy_renamed::ident_id!(1) }>>::Type =
+    |                                                   ^
+    |
+    = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+   --> $DIR/privacy.rs:152:51
+    |
+152 |     let _: <Enum as zerocopy_renamed::HasField<_, _, { zerocopy_renamed::ident_id!(a) }>>::Type =
+    |                                                   ^
+    |
+    = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+  --> $DIR/privacy.rs:31:13
+   |
+31 |             _,
+   |             ^
+   |
+   = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+  --> $DIR/privacy.rs:37:13
+   |
+37 |             _,
+   |             ^
+   |
+   = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+  --> $DIR/privacy.rs:50:13
+   |
+50 |             _,
+   |             ^
+   |
+   = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+  --> $DIR/privacy.rs:56:13
+   |
+56 |             _,
+   |             ^
+   |
+   = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+  --> $DIR/privacy.rs:70:56
+   |
+70 |         let _: <Union as zerocopy_renamed::HasField<_, _, { zerocopy_renamed::ident_id!(a) }>>::Type = 0u8;
+   |                                                        ^
+   |
+   = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+  --> $DIR/privacy.rs:72:56
+   |
+72 |         let _: <Union as zerocopy_renamed::HasField<_, _, { zerocopy_renamed::ident_id!(b) }>>::Type = 0u16;
+   |                                                        ^
+   |
+   = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+  --> $DIR/privacy.rs:85:55
+   |
+85 |         let _: <Enum as zerocopy_renamed::HasField<_, _, { zerocopy_renamed::ident_id!(0) }>>::Type = 0u8;
+   |                                                       ^
+   |
+   = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+  --> $DIR/privacy.rs:87:55
+   |
+87 |         let _: <Enum as zerocopy_renamed::HasField<_, _, { zerocopy_renamed::ident_id!(1) }>>::Type = 0u16;
+   |                                                       ^
+   |
+   = help: const arguments cannot yet be inferred with `_`
+
+error[E0747]: type provided when a constant was expected
+  --> $DIR/privacy.rs:90:55
+   |
+90 |         let _: <Enum as zerocopy_renamed::HasField<_, _, { zerocopy_renamed::ident_id!(a) }>>::Type = 0u8;
+   |                                                       ^
+   |
+   = help: const arguments cannot yet be inferred with `_`
+
+error: aborting due to 18 previous errors
+
+For more information about this error, try `rustc --explain E0747`.