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/darling_core/Cargo.toml

@@ -0,0 +1,68 @@
+# 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 = "1.56"
+name = "darling_core"
+version = "0.21.3"
+authors = ["Ted Driggs <ted.driggs@outlook.com>"]
+build = false
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = """
+Helper crate for proc-macro library for reading attributes into structs when
+implementing custom derives. Use https://crates.io/crates/darling in your code.
+"""
+readme = false
+license = "MIT"
+repository = "https://github.com/TedDriggs/darling"
+
+[features]
+diagnostics = []
+suggestions = ["strsim"]
+
+[lib]
+name = "darling_core"
+path = "src/lib.rs"
+
+[dependencies.fnv]
+version = "1.0.7"
+
+[dependencies.ident_case]
+version = "1.0.1"
+
+[dependencies.proc-macro2]
+version = "1.0.86"
+
+[dependencies.quote]
+version = "1.0.18"
+
+[dependencies.serde]
+version = "1.0.210"
+optional = true
+
+[dependencies.strsim]
+version = "0.11.1"
+optional = true
+
+[dependencies.syn]
+version = "2.0.15"
+features = [
+    "full",
+    "extra-traits",
+]
+
+[dev-dependencies.serde_json]
+version = "1.0.140"