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/bitflags-1.3.2/tests/compile-fail/visibility/private_field.rs

@@ -0,0 +1,13 @@
+mod example {
+    use bitflags::bitflags;
+
+    bitflags! {
+        pub struct Flags1: u32 {
+            const FLAG_A = 0b00000001;
+        }
+    }
+}
+
+fn main() {
+    let flag1 = example::Flags1::FLAG_A.bits;
+}

vendor/bitflags-1.3.2/tests/compile-fail/visibility/private_field.stderr.beta

@@ -0,0 +1,10 @@
+error[E0616]: field `bits` of struct `Flags1` is private
+  --> $DIR/private_field.rs:12:41
+   |
+12 |     let flag1 = example::Flags1::FLAG_A.bits;
+   |                                         ^^^^ private field
+   |
+help: a method `bits` also exists, call it with parentheses
+   |
+12 |     let flag1 = example::Flags1::FLAG_A.bits();
+   |                                             ^^

vendor/bitflags-1.3.2/tests/compile-fail/visibility/private_flags.rs

@@ -0,0 +1,18 @@
+mod example {
+    use bitflags::bitflags;
+
+    bitflags! {
+        pub struct Flags1: u32 {
+            const FLAG_A = 0b00000001;
+        }
+
+        struct Flags2: u32 {
+            const FLAG_B = 0b00000010;
+        }
+    }
+}
+
+fn main() {
+    let flag1 = example::Flags1::FLAG_A;
+    let flag2 = example::Flags2::FLAG_B;
+}

vendor/bitflags-1.3.2/tests/compile-fail/visibility/private_flags.stderr.beta

@@ -0,0 +1,18 @@
+error[E0603]: struct `Flags2` is private
+  --> $DIR/private_flags.rs:17:26
+   |
+17 |     let flag2 = example::Flags2::FLAG_B;
+   |                          ^^^^^^ private struct
+   |
+note: the struct `Flags2` is defined here
+  --> $DIR/private_flags.rs:4:5
+   |
+4  | /     bitflags! {
+5  | |         pub struct Flags1: u32 {
+6  | |             const FLAG_A = 0b00000001;
+7  | |         }
+...  |
+11 | |         }
+12 | |     }
+   | |_____^
+   = note: this error originates in the macro `bitflags` (in Nightly builds, run with -Z macro-backtrace for more info)

vendor/bitflags-1.3.2/tests/compile-fail/visibility/pub_const.rs

@@ -0,0 +1,9 @@
+use bitflags::bitflags;
+
+bitflags! {
+    pub struct Flags1: u32 {
+        pub const FLAG_A = 0b00000001;
+    }
+}
+
+fn main() {}

vendor/bitflags-1.3.2/tests/compile-fail/visibility/pub_const.stderr.beta

@@ -0,0 +1,5 @@
+error: no rules expected the token `pub`
+ --> $DIR/pub_const.rs:5:9
+  |
+5 |         pub const FLAG_A = 0b00000001;
+  |         ^^^ no rules expected this token in macro call