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/README.md

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+bitflags
+========
+
+[![Rust](https://github.com/bitflags/bitflags/workflows/Rust/badge.svg)](https://github.com/bitflags/bitflags/actions)
+[![Latest version](https://img.shields.io/crates/v/bitflags.svg)](https://crates.io/crates/bitflags)
+[![Documentation](https://docs.rs/bitflags/badge.svg)](https://docs.rs/bitflags)
+![License](https://img.shields.io/crates/l/bitflags.svg)
+
+`bitflags` generates flags enums with well-defined semantics and ergonomic end-user APIs.
+
+You can use `bitflags` to:
+
+- provide more user-friendly bindings to C APIs where flags may or may not be fully known in advance.
+- generate efficient options types with string parsing and formatting support.
+
+You can't use `bitflags` to:
+
+- guarantee only bits corresponding to defined flags will ever be set. `bitflags` allows access to the underlying bits type so arbitrary bits may be set.
+- define bitfields. `bitflags` only generates types where set bits denote the presence of some combination of flags.
+
+- [Documentation](https://docs.rs/bitflags)
+- [Specification](https://github.com/bitflags/bitflags/blob/main/spec.md)
+- [Release notes](https://github.com/bitflags/bitflags/releases)
+
+## Usage
+
+Add this to your `Cargo.toml`:
+
+```toml
+[dependencies]
+bitflags = "2.11.0"
+```
+
+and this to your source code:
+
+```rust
+use bitflags::bitflags;
+```
+
+## Example
+
+Generate a flags structure:
+
+```rust
+use bitflags::bitflags;
+
+// The `bitflags!` macro generates `struct`s that manage a set of flags.
+bitflags! {
+    /// Represents a set of flags.
+    #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+    struct Flags: u32 {
+        /// The value `A`, at bit position `0`.
+        const A = 0b00000001;
+        /// The value `B`, at bit position `1`.
+        const B = 0b00000010;
+        /// The value `C`, at bit position `2`.
+        const C = 0b00000100;
+
+        /// The combination of `A`, `B`, and `C`.
+        const ABC = Self::A.bits() | Self::B.bits() | Self::C.bits();
+    }
+}
+
+fn main() {
+    let e1 = Flags::A | Flags::C;
+    let e2 = Flags::B | Flags::C;
+    assert_eq!((e1 | e2), Flags::ABC);   // union
+    assert_eq!((e1 & e2), Flags::C);     // intersection
+    assert_eq!((e1 - e2), Flags::A);     // set difference
+    assert_eq!(!e2, Flags::A);           // set complement
+}
+```
+
+## Cargo features
+
+The `bitflags` library defines a few Cargo features that you can opt-in to:
+
+- `std`: Implement the `Error` trait on error types used by `bitflags`.
+- `serde`: Support deriving `serde` traits on generated flags types.
+- `arbitrary`: Support deriving `arbitrary` traits on generated flags types.
+- `bytemuck`: Support deriving `bytemuck` traits on generated flags types.
+
+Also see [`bitflags_derive`](https://github.com/bitflags/bitflags-derive) for other flags-aware traits.
+
+## Rust Version Support
+
+The minimum supported Rust version is documented in the `Cargo.toml` file.
+This may be bumped in minor releases as necessary.