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/lib.rs

@@ -0,0 +1,145 @@
+//! `static` friendly data structures that don't require dynamic memory allocation
+//!
+//! The core principle behind `heapless` is that its data structures are backed by a *static* memory
+//! allocation. For example, you can think of `heapless::Vec` as an alternative version of
+//! `std::Vec` with fixed capacity and that can't be re-allocated on the fly (e.g. via `push`).
+//!
+//! All `heapless` data structures store their memory allocation *inline* and specify their capacity
+//! via their type parameter `N`. This means that you can instantiate a `heapless` data structure on
+//! the stack, in a `static` variable, or even in the heap.
+//!
+//! ```
+//! use heapless::Vec; // fixed capacity `std::Vec`
+//!
+//! // on the stack
+//! let mut xs: Vec<u8, 8> = Vec::new(); // can hold up to 8 elements
+//! xs.push(42).unwrap();
+//! assert_eq!(xs.pop(), Some(42));
+//!
+//! // in a `static` variable
+//! static mut XS: Vec<u8, 8> = Vec::new();
+//!
+//! let xs = unsafe { &mut XS };
+//!
+//! xs.push(42);
+//! assert_eq!(xs.pop(), Some(42));
+//!
+//! // in the heap (though kind of pointless because no reallocation)
+//! let mut ys: Box<Vec<u8, 8>> = Box::new(Vec::new());
+//! ys.push(42).unwrap();
+//! assert_eq!(ys.pop(), Some(42));
+//! ```
+//!
+//! Because they have fixed capacity `heapless` data structures don't implicitly reallocate. This
+//! means that operations like `heapless::Vec.push` are *truly* constant time rather than amortized
+//! constant time with potentially unbounded (depends on the allocator) worst case execution time
+//! (which is bad / unacceptable for hard real time applications).
+//!
+//! `heapless` data structures don't use a memory allocator which means no risk of an uncatchable
+//! Out Of Memory (OOM) condition while performing operations on them. It's certainly possible to
+//! run out of capacity while growing `heapless` data structures, but the API lets you handle this
+//! possibility by returning a `Result` on operations that may exhaust the capacity of the data
+//! structure.
+//!
+//! List of currently implemented data structures:
+//!
+#![cfg_attr(
+    any(arm_llsc, target_arch = "x86"),
+    doc = "- [`Arc`](pool::arc::Arc) -- like `std::sync::Arc` but backed by a lock-free memory pool rather than `#[global_allocator]`"
+)]
+#![cfg_attr(
+    any(arm_llsc, target_arch = "x86"),
+    doc = "- [`Box`](pool::boxed::Box) -- like `std::boxed::Box` but backed by a lock-free memory pool rather than `#[global_allocator]`"
+)]
+//! - [`BinaryHeap`] -- priority queue
+//! - [`IndexMap`] -- hash table
+//! - [`IndexSet`] -- hash set
+//! - [`LinearMap`]
+#![cfg_attr(
+    any(arm_llsc, target_arch = "x86"),
+    doc = "- [`Object`](pool::object::Object) -- objects managed by an object pool"
+)]
+//! - [`String`]
+//! - [`Vec`]
+//! - [`mpmc::Q*`](mpmc) -- multiple producer multiple consumer lock-free queue
+//! - [`spsc::Queue`] -- single producer single consumer lock-free queue
+//!
+//! # Optional Features
+//!
+//! The `heapless` crate provides the following optional Cargo features:
+//!
+//! - `ufmt`: Implement [`ufmt_write::uWrite`] for `String<N>` and `Vec<u8, N>`
+//!
+//! [`ufmt_write::uWrite`]: https://docs.rs/ufmt-write/
+//!
+//! # Minimum Supported Rust Version (MSRV)
+//!
+//! This crate does *not* have a Minimum Supported Rust Version (MSRV) and may make use of language
+//! features and API in the standard library available in the latest stable Rust version.
+//!
+//! In other words, changes in the Rust version requirement of this crate are not considered semver
+//! breaking change and may occur in patch version releases.
+#![cfg_attr(docsrs, feature(doc_cfg), feature(doc_auto_cfg))]
+#![cfg_attr(not(test), no_std)]
+#![deny(missing_docs)]
+#![deny(warnings)]
+
+pub use binary_heap::BinaryHeap;
+pub use deque::Deque;
+pub use histbuf::{HistoryBuffer, OldestOrdered};
+pub use indexmap::{
+    Bucket, Entry, FnvIndexMap, IndexMap, Iter as IndexMapIter, IterMut as IndexMapIterMut,
+    Keys as IndexMapKeys, OccupiedEntry, Pos, VacantEntry, Values as IndexMapValues,
+    ValuesMut as IndexMapValuesMut,
+};
+pub use indexset::{FnvIndexSet, IndexSet, Iter as IndexSetIter};
+pub use linear_map::LinearMap;
+pub use string::String;
+pub use vec::Vec;
+
+#[macro_use]
+#[cfg(test)]
+mod test_helpers;
+
+mod deque;
+mod histbuf;
+mod indexmap;
+mod indexset;
+mod linear_map;
+mod string;
+mod vec;
+
+#[cfg(feature = "serde")]
+mod de;
+#[cfg(feature = "serde")]
+mod ser;
+
+pub mod binary_heap;
+#[cfg(feature = "defmt-03")]
+mod defmt;
+#[cfg(any(
+    // assume we have all atomics available if we're using portable-atomic
+    feature = "portable-atomic",
+    // target has native atomic CAS (mpmc_large requires usize, otherwise just u8)
+    all(feature = "mpmc_large", target_has_atomic = "ptr"),
+    all(not(feature = "mpmc_large"), target_has_atomic = "8")
+))]
+pub mod mpmc;
+#[cfg(any(arm_llsc, target_arch = "x86"))]
+pub mod pool;
+pub mod sorted_linked_list;
+#[cfg(any(
+    // assume we have all atomics available if we're using portable-atomic
+    feature = "portable-atomic",
+    // target has native atomic CAS. Note this is too restrictive, spsc requires load/store only, not CAS.
+    // This should be `cfg(target_has_atomic_load_store)`, but that's not stable yet.
+    target_has_atomic = "ptr",
+    // or the current target is in a list in build.rs of targets known to have load/store but no CAS.
+    has_atomic_load_store
+))]
+pub mod spsc;
+
+#[cfg(feature = "ufmt")]
+mod ufmt;
+
+mod sealed;