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/spin/Cargo.toml

@@ -0,0 +1,80 @@
+# 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 = "1.38"
+name = "spin"
+version = "0.9.8"
+authors = [
+    "Mathijs van de Nes <git@mathijs.vd-nes.nl>",
+    "John Ericson <git@JohnEricson.me>",
+    "Joshua Barretto <joshua.s.barretto@gmail.com>",
+]
+description = "Spin-based synchronization primitives"
+readme = "README.md"
+keywords = [
+    "spinlock",
+    "mutex",
+    "rwlock",
+]
+license = "MIT"
+repository = "https://github.com/mvdnes/spin-rs.git"
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = [
+    "--cfg",
+    "docsrs",
+]
+
+[[bench]]
+name = "mutex"
+harness = false
+required-features = ["ticket_mutex"]
+
+[dependencies.lock_api_crate]
+version = "0.4"
+optional = true
+package = "lock_api"
+
+[dependencies.portable-atomic]
+version = "1"
+optional = true
+default-features = false
+
+[dev-dependencies.criterion]
+version = "0.4"
+
+[features]
+barrier = ["mutex"]
+default = [
+    "lock_api",
+    "mutex",
+    "spin_mutex",
+    "rwlock",
+    "once",
+    "lazy",
+    "barrier",
+]
+fair_mutex = ["mutex"]
+lazy = ["once"]
+lock_api = ["lock_api_crate"]
+mutex = []
+once = []
+portable_atomic = ["portable-atomic"]
+rwlock = []
+spin_mutex = ["mutex"]
+std = []
+ticket_mutex = ["mutex"]
+use_ticket_mutex = [
+    "mutex",
+    "ticket_mutex",
+]