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/toml_parser/src/lib.rs

@@ -0,0 +1,49 @@
+//! TOML lexer and parser
+//!
+//! Characteristics:
+//! - Error recovery
+//! - Lazy validation
+//! - `forbid(unsafe)` by default, requiring the `unsafe` feature otherwise
+//! - `no_std` support, including putting users in charge of allocation choices (including not
+//!   allocating)
+//!
+//! Full parsing is broken into three phases:
+//! 1. [Lexing tokens][lexer]
+//! 2. [Parsing tokens][parser] (push parser)
+//! 3. Organizing the physical layout into the logical layout,
+//!    including [decoding keys and values][decoder]
+
+#![cfg_attr(all(not(feature = "std"), not(test)), no_std)]
+#![cfg_attr(docsrs, feature(doc_cfg))]
+#![cfg_attr(not(feature = "unsafe"), forbid(unsafe_code))]
+#![warn(clippy::std_instead_of_core)]
+#![warn(clippy::std_instead_of_alloc)]
+#![warn(clippy::print_stderr)]
+#![warn(clippy::print_stdout)]
+
+#[cfg(feature = "alloc")]
+extern crate alloc;
+
+#[macro_use]
+mod macros;
+
+#[cfg(feature = "debug")]
+pub(crate) mod debug;
+mod error;
+mod source;
+
+pub mod decoder;
+pub mod lexer;
+pub mod parser;
+
+pub use error::ErrorSink;
+pub use error::Expected;
+pub use error::ParseError;
+pub use source::Raw;
+pub use source::Source;
+pub use source::SourceIndex;
+pub use source::Span;
+
+#[doc = include_str!("../README.md")]
+#[cfg(doctest)]
+pub struct ReadmeDoctests;