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/zerocopy/rustfmt.toml

@@ -0,0 +1,22 @@
+# Copyright 2022 The Fuchsia Authors
+#
+# Licensed under a BSD-style license <LICENSE-BSD>, Apache License, Version 2.0
+# <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
+# license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
+# This file may not be copied, modified, or distributed except according to
+# those terms.
+
+edition = "2024"
+
+# The "Default" setting has a heuristic which splits lines too aggressively.
+# We are willing to revisit this setting in future versions of rustfmt.
+# Bugs:
+#   * https://github.com/rust-lang/rustfmt/issues/3119
+#   * https://github.com/rust-lang/rustfmt/issues/3120
+use_small_heuristics = "Max"
+
+# Prevent carriage returns
+newline_style = "Unix"
+
+imports_granularity = "Crate"
+group_imports = "StdExternalCrate"