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/scopeguard/Cargo.toml

@@ -0,0 +1,43 @@
+# 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]
+name = "scopeguard"
+version = "1.2.0"
+authors = ["bluss"]
+description = """
+A RAII scope guard that will run a given closure when it goes out of scope,
+even if the code between panics (assuming unwinding panic).
+
+Defines the macros `defer!`, `defer_on_unwind!`, `defer_on_success!` as
+shorthands for guards with one of the implemented strategies.
+"""
+documentation = "https://docs.rs/scopeguard/"
+readme = "README.md"
+keywords = [
+    "scope-guard",
+    "defer",
+    "panic",
+    "unwind",
+]
+categories = [
+    "rust-patterns",
+    "no-std",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/bluss/scopeguard"
+
+[package.metadata.release]
+no-dev-version = true
+
+[features]
+default = ["use_std"]
+use_std = []