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/fnv/README.md

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+# rust-fnv
+
+An implementation of the [Fowler–Noll–Vo hash function][chongo].
+
+### [Read the documentation](https://doc.servo.org/fnv/)
+
+
+## About
+
+The FNV hash function is a custom `Hasher` implementation that is more
+efficient for smaller hash keys.
+
+[The Rust FAQ states that][faq] while the default `Hasher` implementation,
+SipHash, is good in many cases, it is notably slower than other algorithms
+with short keys, such as when you have a map of integers to other values.
+In cases like these, [FNV is demonstrably faster][graphs].
+
+Its disadvantages are that it performs badly on larger inputs, and
+provides no protection against collision attacks, where a malicious user
+can craft specific keys designed to slow a hasher down. Thus, it is
+important to profile your program to ensure that you are using small hash
+keys, and be certain that your program could not be exposed to malicious
+inputs (including being a networked server).
+
+The Rust compiler itself uses FNV, as it is not worried about
+denial-of-service attacks, and can assume that its inputs are going to be
+small—a perfect use case for FNV.
+
+
+## Usage
+
+To include this crate in your program, add the following to your `Cargo.toml`:
+
+```toml
+[dependencies]
+fnv = "1.0.3"
+```
+
+
+## Using FNV in a HashMap
+
+The `FnvHashMap` type alias is the easiest way to use the standard library’s
+`HashMap` with FNV.
+
+```rust
+use fnv::FnvHashMap;
+
+let mut map = FnvHashMap::default();
+map.insert(1, "one");
+map.insert(2, "two");
+
+map = FnvHashMap::with_capacity_and_hasher(10, Default::default());
+map.insert(1, "one");
+map.insert(2, "two");
+```
+
+Note, the standard library’s `HashMap::new` and `HashMap::with_capacity`
+are only implemented for the `RandomState` hasher, so using `Default` to
+get the hasher is the next best option.
+
+
+## Using FNV in a HashSet
+
+Similarly, `FnvHashSet` is a type alias for the standard library’s `HashSet`
+with FNV.
+
+```rust
+use fnv::FnvHashSet;
+
+let mut set = FnvHashSet::default();
+set.insert(1);
+set.insert(2);
+
+set = FnvHashSet::with_capacity_and_hasher(10, Default::default());
+set.insert(1);
+set.insert(2);
+```
+
+[chongo]: http://www.isthe.com/chongo/tech/comp/fnv/index.html
+[faq]: https://www.rust-lang.org/en-US/faq.html#why-are-rusts-hashmaps-slow
+[graphs]: https://cglab.ca/~abeinges/blah/hash-rs/