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/riscv/src/lib.rs

@@ -0,0 +1,62 @@
+//! Low level access to RISC-V processors
+//!
+//! # Minimum Supported Rust Version (MSRV)
+//!
+//! This crate is guaranteed to compile on stable Rust 1.60 and up. It *might*
+//! compile with older versions but that may change in any new patch release.
+//!
+//! # Features
+//!
+//! This crate provides:
+//!
+//! - Access to core registers like `mstatus` or `mcause`.
+//! - Interrupt manipulation mechanisms.
+//! - Wrappers around assembly instructions like `WFI`.
+//!
+//! # Optional features
+//!
+//! ## `s-mode`
+//!
+//! This feature re-exports in `interrupt` S-mode interrupt functions defined in `interrupt::supervisor`.
+//! By default, the crate assumes that the target is running in M-mode.
+//! Thus, `interrupt` re-exports the M-mode functions defined in `interrupt::machine`.
+//!
+//! ## `critical-section-single-hart`
+//!
+//! This feature enables a [`critical-section`](https://github.com/rust-embedded/critical-section)
+//! implementation suitable for single-hart targets, based on disabling interrupts globally.
+//! This feature uses S-mode interrupt handling if the `s-mode` feature is enabled, and M-mode otherwise.
+//!
+//! It is **unsound** to enable it on multi-hart targets,
+//! and may cause functional problems in systems where some interrupts must NOT be disabled
+//! or critical sections are managed as part of an RTOS. In these cases, you should use
+//! a target-specific implementation instead, typically provided by a HAL or RTOS crate.
+
+#![no_std]
+#![allow(clippy::missing_safety_doc)]
+
+pub use paste::paste;
+
+pub mod asm;
+pub mod bits;
+pub mod delay;
+pub mod interrupt;
+pub mod register;
+
+// Re-export crates of the RISC-V ecosystem
+#[cfg(feature = "riscv-macros")]
+pub use riscv_macros::*;
+pub use riscv_pac::*;
+
+#[macro_use]
+mod macros;
+
+#[cfg(all(riscv, feature = "critical-section-single-hart"))]
+mod critical_section;
+
+/// Used to reexport items for use in macros. Do not use directly.
+/// Not covered by semver guarantees.
+#[doc(hidden)]
+pub mod _export {
+    pub use critical_section;
+}