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/serde_derive/Cargo.toml

@@ -0,0 +1,87 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies.
+#
+# If you are reading this file be aware that the original Cargo.toml
+# will likely look very different (and much more reasonable).
+# See Cargo.toml.orig for the original contents.
+
+[package]
+edition = "2021"
+rust-version = "1.61"
+name = "serde_derive"
+version = "1.0.228"
+authors = [
+    "Erick Tryzelaar <erick.tryzelaar@gmail.com>",
+    "David Tolnay <dtolnay@gmail.com>",
+]
+build = false
+exclude = ["build.rs"]
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "Macros 1.1 implementation of #[derive(Serialize, Deserialize)]"
+homepage = "https://serde.rs"
+documentation = "https://serde.rs/derive.html"
+readme = "crates-io.md"
+keywords = [
+    "serde",
+    "serialization",
+    "no_std",
+    "derive",
+]
+categories = [
+    "no-std",
+    "no-std::no-alloc",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/serde-rs/serde"
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]
+rustdoc-args = [
+    "--generate-link-to-definition",
+    "--generate-macro-expansion",
+    "--extern-html-root-url=core=https://doc.rust-lang.org",
+    "--extern-html-root-url=alloc=https://doc.rust-lang.org",
+    "--extern-html-root-url=std=https://doc.rust-lang.org",
+    "--extern-html-root-url=proc_macro=https://doc.rust-lang.org",
+]
+
+[features]
+default = []
+deserialize_in_place = []
+
+[lib]
+name = "serde_derive"
+path = "src/lib.rs"
+proc-macro = true
+
+[dependencies.proc-macro2]
+version = "1.0.74"
+features = ["proc-macro"]
+default-features = false
+
+[dependencies.quote]
+version = "1.0.35"
+features = ["proc-macro"]
+default-features = false
+
+[dependencies.syn]
+version = "2.0.81"
+features = [
+    "clone-impls",
+    "derive",
+    "parsing",
+    "printing",
+    "proc-macro",
+]
+default-features = false
+
+[dev-dependencies.serde]
+version = "1"