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/autocfg/Cargo.toml

@@ -0,0 +1,76 @@
+# 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]
+rust-version = "1.0"
+name = "autocfg"
+version = "1.5.0"
+authors = ["Josh Stone <cuviper@gmail.com>"]
+build = false
+exclude = ["/.github/**"]
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "Automatic cfg for Rust compiler features"
+documentation = "https://docs.rs/autocfg/"
+readme = "README.md"
+keywords = [
+    "rustc",
+    "build",
+    "autoconf",
+]
+categories = ["development-tools::build-utils"]
+license = "Apache-2.0 OR MIT"
+repository = "https://github.com/cuviper/autocfg"
+
+[lib]
+name = "autocfg"
+path = "src/lib.rs"
+
+[[example]]
+name = "integers"
+path = "examples/integers.rs"
+
+[[example]]
+name = "nightly"
+path = "examples/nightly.rs"
+
+[[example]]
+name = "paths"
+path = "examples/paths.rs"
+
+[[example]]
+name = "traits"
+path = "examples/traits.rs"
+
+[[example]]
+name = "versions"
+path = "examples/versions.rs"
+
+[[test]]
+name = "no_std"
+path = "tests/no_std.rs"
+
+[[test]]
+name = "rustflags"
+path = "tests/rustflags.rs"
+
+[[test]]
+name = "tests"
+path = "tests/tests.rs"
+
+[[test]]
+name = "wrappers"
+path = "tests/wrappers.rs"
+
+[dependencies]