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/ssmarshal/README.md

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+# ssmarshal
+
+[![Crates.io](https://img.shields.io/crates/v/ssmarshal.svg?style=flat-square)](https://crates.io/crates/ssmarshal)
+
+[Documentation](https://docs.rs/ssmarshal)
+
+ssmarshal ("stupid simple marshaling") is a serde-based de/serialization
+library. It is somewhat like [bincode](https://github.com/TyOverby/bincode),
+but doesn't support String/Vec - this library is entirely zero-allocation, for
+use in extremely limited `no_std` contexts. The key invariant is that the
+encoding space for any value of a type is `<= size_of::<T>()`. This allows
+easy reasoning about limited buffer sizes, and how much is always enough.
+
+## Limitations
+
+These sorts of types are not supported:
+
+- Any non-core type (eg, `Vec`, `HashMap`)
+- Slices
+- Strings
+- Types containing references or pointers
+- Enums with more than 256 variants
+
+All enums MUST be `#[repr(C)]` in order for the size invariant to be upheld.
+Note that this excludes using `Option`, especially with `NonZero` types!
+You can use this crate with non-`#[repr(C)]` enums, but you should thoroughly
+test (I recommend quickcheck, see [the tests](https://gitlab.com/robigalia/ssmarshal/blob/master/tests/roundtrip.rs)
+for an example) de/serializing values of that type to be assured the size
+invariant holds for the type.
+
+## Details of the format
+
+The format is not incredibly compact, but doesn't add extra fluff, and is
+quick to en/decode. 
+
+- `bool` is serialized as a byte, 1 if true, 0 is false.
+- the integer types are encoded in their little-endian form.
+- f32 is bitcast to a u32 then encoded as a u32. likewise, f64 and u64.
+- inhabited enums are serialized as 1 byte for the discriminant, and then the fields.
+- structs are serialized as just their fields.
+- the unit type and uninhabited enums are not serialized at all.
+- tuples are serialized as the fields, in order.
+
+There is no padding.
+
+As you might see, this format is not self-describing. To successfully
+deserialize a value, the exact layout must be known ahead-of-time.
+
+## Alternatives
+
+This is designed for doing IPC in a microkernel, with a stable ABI, not saving
+to disk or transferring over the network. It may be useful for those cases,
+although you'll likely want a format which can handle data evolution, like
+Cap'n Proto.
+
+If you care about truly minimizing encoding space, you might look into ASN.1
+PER. 
+
+If you need more features (for example, slices or references), but still don't
+care about data evolution, I recommend using bincode.
+
+## Testing
+
+The `roundtrip` test suite exercises almost all of the functionality of
+ssmarshal, with coverage over 95% (missing mostly some of the error cases for
+invalid/unsupported types).
+
+This library is extensively fuzz tested with `cargo-fuzz` (libFuzzer) before
+every release. See the `fuzz` directory for the scripts used.