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/embassy-hal-internal/Cargo.toml

@@ -0,0 +1,69 @@
+# 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"
+name = "embassy-hal-internal"
+version = "0.2.0"
+build = "build.rs"
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "Internal implementation details for Embassy HALs. DO NOT USE DIRECTLY."
+documentation = "https://docs.embassy.dev/embassy-hal-internal"
+readme = "README.md"
+categories = [
+    "embedded",
+    "no-std",
+    "asynchronous",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/embassy-rs/embassy"
+
+[lib]
+name = "embassy_hal_internal"
+path = "src/lib.rs"
+
+[dependencies.cortex-m]
+version = "0.7.6"
+optional = true
+
+[dependencies.critical-section]
+version = "1"
+optional = true
+
+[dependencies.defmt]
+version = "0.3"
+optional = true
+
+[dependencies.log]
+version = "0.4.14"
+optional = true
+
+[dependencies.num-traits]
+version = "0.2.14"
+default-features = false
+
+[features]
+cortex-m = [
+    "dep:cortex-m",
+    "dep:critical-section",
+]
+prio-bits-0 = []
+prio-bits-1 = []
+prio-bits-2 = []
+prio-bits-3 = []
+prio-bits-4 = []
+prio-bits-5 = []
+prio-bits-6 = []
+prio-bits-7 = []
+prio-bits-8 = []