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_write/src/lib.rs

@@ -0,0 +1,81 @@
+//! A low-level interface for writing out TOML
+//!
+//! Considerations when serializing arbitrary data:
+//! - Verify the implementation with [`toml-test-harness`](https://docs.rs/toml-test-harness)
+//! - Be sure to group keys under a table before writing another table
+//! - Watch for extra trailing newlines and leading newlines, both when starting with top-level
+//!   keys or a table
+//! - When serializing an array-of-tables, be sure to verify that all elements of the array
+//!   serialize as tables
+//! - Standard tables and inline tables may need separate implementations of corner cases,
+//!   requiring verifying them both
+//!
+//! When serializing Rust data structures
+//! - `Option`: Skip key-value pairs with a value of `None`, otherwise error when seeing `None`
+//!   - When skipping key-value pairs, be careful that a deeply nested `None` doesn't get skipped
+//! - Scalars and arrays are unsupported as top-level data types
+//! - Tuples and tuple variants seriallize as arrays
+//! - Structs, struct variants, and maps serialize as tables
+//! - Newtype variants serialize as to the inner type
+//! - Unit variants serialize to a string
+//! - Unit and unit structs don't have a clear meaning in TOML
+//!
+//! # Example
+//!
+//! ```rust
+//! use toml_write::TomlWrite as _;
+//!
+//! # fn main() -> std::fmt::Result {
+//! let mut output = String::new();
+//! output.newline()?;
+//! output.open_table_header()?;
+//! output.key("table")?;
+//! output.close_table_header()?;
+//! output.newline()?;
+//!
+//! output.key("key")?;
+//! output.space()?;
+//! output.keyval_sep()?;
+//! output.space()?;
+//! output.value("value")?;
+//! output.newline()?;
+//!
+//! assert_eq!(output, r#"
+//! [table]
+//! key = "value"
+//! "#);
+//! #   Ok(())
+//! # }
+//! ```
+
+#![cfg_attr(all(not(feature = "std"), not(test)), no_std)]
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![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;
+
+mod key;
+mod string;
+mod value;
+mod write;
+
+#[cfg(feature = "alloc")]
+pub use key::ToTomlKey;
+pub use key::WriteTomlKey;
+pub use string::TomlKey;
+pub use string::TomlKeyBuilder;
+pub use string::TomlString;
+pub use string::TomlStringBuilder;
+#[cfg(feature = "alloc")]
+pub use value::ToTomlValue;
+pub use value::WriteTomlValue;
+pub use write::TomlWrite;
+
+#[doc = include_str!("../README.md")]
+#[cfg(doctest)]
+pub struct ReadmeDoctests;