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/usbd-hid-descriptors/src/lib.rs

@@ -0,0 +1,125 @@
+#![no_std]
+
+use bitfield::bitfield;
+
+/// GlobalItemKind describes global item tags as described in section 6.2.2.7
+/// 'Report Descriptor' of the spec, version 1.11.
+#[repr(u8)]
+#[allow(unused)]
+#[derive(Copy, Debug, Clone, Eq, PartialEq)]
+pub enum GlobalItemKind {
+    UsagePage = 0,
+    LogicalMin = 1,
+    LogicalMax = 2,
+    PhysicalMin = 3,
+    PhysicalMax = 4,
+    UnitExponent = 5,
+    Unit = 6,
+    ReportSize = 7,
+    ReportID = 8,
+    ReportCount = 9,
+}
+
+impl From<GlobalItemKind> for u8 {
+    fn from(kind: GlobalItemKind) -> u8 {
+        kind as u8
+    }
+}
+
+/// LocalItemKind describes local item tags as described in section 6.2.2.8
+/// 'Local Items' of the spec, version 1.11.
+#[repr(u8)]
+#[allow(unused)]
+#[derive(Copy, Debug, Clone, Eq, PartialEq)]
+pub enum LocalItemKind {
+    Usage = 0,
+    UsageMin = 1,
+    UsageMax = 2,
+    DesignatorIdx = 3,
+    DesignatorMin = 4,
+    DesignatorMax = 5,
+    StringIdx = 7,
+    StringMin = 8,
+    StringMax = 9,
+    Delimiter = 10,
+}
+
+impl From<LocalItemKind> for u8 {
+    fn from(kind: LocalItemKind) -> u8 {
+        kind as u8
+    }
+}
+
+/// MainItemKind describes main item tags as described in section 6.2.2.4
+/// 'Report Descriptor' of the spec, version 1.11.
+#[repr(u8)]
+#[allow(unused)]
+#[derive(Copy, Debug, Default, Clone, Eq, PartialEq)]
+pub enum MainItemKind {
+    #[default]
+    Input = 0b1000,
+    Output = 0b1001,
+    Feature = 0b1011,
+    Collection = 0b1010,
+    EndCollection = 0b1100,
+}
+
+impl From<MainItemKind> for u8 {
+    fn from(kind: MainItemKind) -> u8 {
+        kind as u8
+    }
+}
+
+impl From<&str> for MainItemKind {
+    fn from(s: &str) -> Self {
+        match s {
+            "feature" => MainItemKind::Feature,
+            "output" => MainItemKind::Output,
+            "collection" => MainItemKind::Collection,
+            "ecollection" => MainItemKind::EndCollection,
+            "input" => MainItemKind::Input,
+            _ => MainItemKind::Input,
+        }
+    }
+}
+
+/// ItemType describes types of items as described in section 6.2.2.7
+/// 'Report Descriptor' of the spec, version 1.11.
+#[repr(u8)]
+#[allow(unused)]
+#[derive(Copy, Debug, Default, Clone, Eq, PartialEq)]
+pub enum ItemType {
+    #[default]
+    Main = 0,
+    Global = 1,
+    Local = 2,
+}
+
+impl From<ItemType> for u8 {
+    fn from(kind: ItemType) -> u8 {
+        kind as u8
+    }
+}
+
+bitfield! {
+    /// MainItemSetting describes the bits which configure invariants on a MainItem.
+    #[derive(Clone,Debug)]
+    pub struct MainItemSetting(u8);
+    pub is_constant, set_constant: 0;
+    pub is_variable, set_variable: 1;
+    pub is_relative, set_relative: 2;
+    pub is_wrap, set_wrap: 3;
+    pub is_non_linear, set_non_linear: 4;
+    pub has_no_preferred_state, set_no_preferred_state: 5;
+    pub has_null_state, set_has_null_state: 6;
+    pub volatile, set_volatile: 7;
+}
+
+bitfield! {
+    /// ItemPrefix describes the 1 byte prefix describing an item in a descriptor.
+    pub struct ItemPrefix(u8);
+    impl Debug;
+    pub byte_count, set_byte_count: 1, 0;
+    pub typ, set_type: 3, 2;
+    pub tag, set_tag: 7, 4;
+}