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/defmt/tests/ui/derive-empty-attr.rs

@@ -0,0 +1,7 @@
+#[derive(defmt::Format)]
+struct S {
+    #[defmt()]
+    f: bool,
+}
+
+fn main() {}

vendor/defmt/tests/ui/derive-empty-attr.stderr

@@ -0,0 +1,5 @@
+error: expected 1 attribute argument
+ --> $DIR/derive-empty-attr.rs:3:7
+  |
+3 |     #[defmt()]
+  |       ^^^^^^^

vendor/defmt/tests/ui/derive-invalid-attr-arg-item.rs

@@ -0,0 +1,7 @@
+#[derive(defmt::Format)]
+#[defmt(FooBar)]
+struct S {
+    f: bool,
+}
+
+fn main() {}

vendor/defmt/tests/ui/derive-invalid-attr-arg-item.stderr

@@ -0,0 +1,5 @@
+error: unknown defmt attribute `FooBar`
+ --> tests/ui/derive-invalid-attr-arg-item.rs:2:9
+  |
+2 | #[defmt(FooBar)]
+  |         ^^^^^^

vendor/defmt/tests/ui/derive-invalid-attr-arg.rs

@@ -0,0 +1,7 @@
+#[derive(defmt::Format)]
+struct S {
+    #[defmt(FooBar)]
+    f: bool,
+}
+
+fn main() {}

vendor/defmt/tests/ui/derive-invalid-attr-arg.stderr

@@ -0,0 +1,5 @@
+error: expected `Debug2Format` or `Display2Format`
+ --> $DIR/derive-invalid-attr-arg.rs:3:13
+  |
+3 |     #[defmt(FooBar)]
+  |             ^^^^^^

vendor/defmt/tests/ui/derive-invalid-crate-overwrite.rs

@@ -0,0 +1,13 @@
+#[derive(defmt::Format)]
+#[defmt(crate = unresolved)]
+struct S {}
+
+#[derive(defmt::Format)]
+#[defmt(crate = "not a path")]
+struct S2 {}
+
+#[derive(defmt::Format)]
+#[defmt(crate(defmt))]
+struct S3 {}
+
+fn main() {}

vendor/defmt/tests/ui/derive-invalid-crate-overwrite.stderr

@@ -0,0 +1,25 @@
+error: expected identifier
+ --> tests/ui/derive-invalid-crate-overwrite.rs:6:17
+  |
+6 | #[defmt(crate = "not a path")]
+  |                 ^^^^^^^^^^^^
+
+error: expected `=`
+  --> tests/ui/derive-invalid-crate-overwrite.rs:10:14
+   |
+10 | #[defmt(crate(defmt))]
+   |              ^
+
+error[E0432]: unresolved import `unresolved`
+ --> tests/ui/derive-invalid-crate-overwrite.rs:1:10
+  |
+1 | #[derive(defmt::Format)]
+  |          ^^^^^^^^^^^^^ no external crate `unresolved`
+  |
+  = note: this error originates in the derive macro `defmt::Format` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0433]: failed to resolve: use of undeclared crate or module `unresolved`
+ --> tests/ui/derive-invalid-crate-overwrite.rs:2:17
+  |
+2 | #[defmt(crate = unresolved)]
+  |                 ^^^^^^^^^^ use of undeclared crate or module `unresolved`

vendor/defmt/tests/ui/derive-multi-attr.rs

@@ -0,0 +1,8 @@
+#[derive(defmt::Format)]
+struct S {
+    #[defmt(Debug2Format)]
+    #[defmt()]
+    f: bool,
+}
+
+fn main() {}

vendor/defmt/tests/ui/derive-multi-attr.stderr

@@ -0,0 +1,7 @@
+error: multiple `defmt` attributes not supported
+ --> $DIR/derive-multi-attr.rs:3:5
+  |
+3 | /     #[defmt(Debug2Format)]
+4 | |     #[defmt()]
+5 | |     f: bool,
+  | |___________^

vendor/defmt/tests/ui/log-invalid-hint.rs

@@ -0,0 +1,3 @@
+fn main() {
+    defmt::info!("{=u8:dunno}", 42)
+}

vendor/defmt/tests/ui/log-invalid-hint.stderr

@@ -0,0 +1,5 @@
+error: unknown display hint: "dunno"
+ --> $DIR/log-invalid-hint.rs:2:18
+  |
+2 |     defmt::info!("{=u8:dunno}", 42)
+  |                  ^^^^^^^^^^^^^

vendor/defmt/tests/ui/panic-handler-argument.rs

@@ -0,0 +1,5 @@
+#![no_main]
+#![no_std]
+
+#[defmt::panic_handler]
+fn foo(x: bool) -> ! { info!("{:?}", x); loop {} }

vendor/defmt/tests/ui/panic-handler-argument.stderr

@@ -0,0 +1,5 @@
+error: function must have signature `fn() -> !`
+ --> $DIR/panic-handler-argument.rs:5:4
+  |
+5 | fn foo(x: bool) -> ! { info!("{:?}", x); loop {} }
+  |    ^^^

vendor/defmt/tests/ui/panic-handler-empty.rs

@@ -0,0 +1,5 @@
+#![no_main]
+#![no_std]
+
+#[defmt::panic_handler]
+fn foo() {}

vendor/defmt/tests/ui/panic-handler-empty.stderr

@@ -0,0 +1,5 @@
+error: function must have signature `fn() -> !`
+ --> $DIR/panic-handler-empty.rs:5:4
+  |
+5 | fn foo() {}
+  |    ^^^

vendor/defmt/tests/ui/panic-handler-export-name.rs

@@ -0,0 +1,8 @@
+#![no_main]
+#![no_std]
+
+#[defmt::panic_handler]
+#[export_name = "hello"]
+fn foo() -> ! {
+    loop {}
+}

vendor/defmt/tests/ui/panic-handler-export-name.stderr

@@ -0,0 +1,5 @@
+error: `#[panic_handler]` attribute cannot be used together with `#[export_name]`
+ --> $DIR/panic-handler-export-name.rs:5:1
+  |
+5 | #[export_name = "hello"]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^

vendor/defmt/tests/ui/panic-handler-no-mangle.rs

@@ -0,0 +1,8 @@
+#![no_main]
+#![no_std]
+
+#[defmt::panic_handler]
+#[no_mangle]
+fn panic() -> ! {
+    loop {}
+}

vendor/defmt/tests/ui/panic-handler-no-mangle.stderr

@@ -0,0 +1,5 @@
+error: `#[panic_handler]` attribute cannot be used together with `#[no_mangle]`
+ --> $DIR/panic-handler-no-mangle.rs:5:1
+  |
+5 | #[no_mangle]
+  | ^^^^^^^^^^^^

vendor/defmt/tests/ui/write-invalid-hint.rs

@@ -0,0 +1,9 @@
+struct S;
+
+impl defmt::Format for S {
+    fn format(&self, f: defmt::Formatter) {
+        defmt::write!(f, "{=u8:dunno}", 42)
+    }
+}
+
+fn main() {}

vendor/defmt/tests/ui/write-invalid-hint.stderr

@@ -0,0 +1,5 @@
+error: unknown display hint: "dunno"
+ --> $DIR/write-invalid-hint.rs:5:26
+  |
+5 |         defmt::write!(f, "{=u8:dunno}", 42)
+  |                          ^^^^^^^^^^^^^