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/heapless/src/pool.rs

@@ -0,0 +1,59 @@
+//! Memory and object pools
+//!
+//! # Target support
+//!
+//! This module / API is only available on these compilation targets:
+//!
+//! - ARM architectures which instruction set include the LDREX, CLREX and STREX instructions, e.g.
+//! `thumbv7m-none-eabi` but not `thumbv6m-none-eabi`
+//! - 32-bit x86, e.g. `i686-unknown-linux-gnu`
+//!
+//! # Benchmarks
+//!
+//! - compilation settings
+//!   - `codegen-units = 1`
+//!   - `lto = 'fat'`
+//!   - `opt-level = 'z'`
+//! - compilation target: `thumbv7em-none-eabihf`
+//! - CPU: ARM Cortex-M4F
+//!
+//! - test program:
+//!
+//! ``` no_run
+//! use heapless::box_pool;
+//!
+//! box_pool!(P: ()); // or `arc_pool!` or `object_pool!`
+//!
+//! bkpt();
+//! let res = P.alloc(());
+//! bkpt();
+//!
+//! if let Ok(boxed) = res {
+//!     bkpt();
+//!     drop(boxed);
+//!     bkpt();
+//! }
+//! # fn bkpt() {}
+//! ```
+//!
+//! - measurement method: the cycle counter (CYCCNT) register was sampled each time a breakpoint
+//! (`bkpt`) was hit. the difference between the "after" and the "before" value of CYCCNT yields the
+//! execution time in clock cycles.
+//!
+//! | API                          | clock cycles |
+//! |------------------------------|--------------|
+//! | `BoxPool::alloc`             | 23           |
+//! | `pool::boxed::Box::drop`     | 23           |
+//! | `ArcPool::alloc`             | 28           |
+//! | `pool::arc::Arc::drop`       | 59           |
+//! | `ObjectPool::request`        | 23           |
+//! | `pool::object::Object::drop` | 23           |
+//!
+//! Note that the execution time won't include `T`'s initialization nor `T`'s destructor which will
+//! be present in the general case for `Box` and `Arc`.
+
+mod treiber;
+
+pub mod arc;
+pub mod boxed;
+pub mod object;