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/hashbrown/Cargo.toml

@@ -0,0 +1,180 @@
+# 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 = "1.65.0"
+name = "hashbrown"
+version = "0.16.1"
+authors = ["Amanieu d'Antras <amanieu@gmail.com>"]
+build = false
+exclude = [
+    ".github",
+    "/ci/*",
+]
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "A Rust port of Google's SwissTable hash map"
+readme = "README.md"
+keywords = [
+    "hash",
+    "no_std",
+    "hashmap",
+    "swisstable",
+]
+categories = [
+    "data-structures",
+    "no-std",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/rust-lang/hashbrown"
+
+[package.metadata.docs.rs]
+features = [
+    "nightly",
+    "rayon",
+    "serde",
+    "raw-entry",
+]
+rustdoc-args = ["--generate-link-to-definition"]
+
+[features]
+default = [
+    "default-hasher",
+    "inline-more",
+    "allocator-api2",
+    "equivalent",
+    "raw-entry",
+]
+default-hasher = ["dep:foldhash"]
+inline-more = []
+nightly = [
+    "foldhash?/nightly",
+    "bumpalo/allocator_api",
+]
+raw-entry = []
+rustc-dep-of-std = [
+    "nightly",
+    "core",
+    "alloc",
+    "rustc-internal-api",
+]
+rustc-internal-api = []
+serde = [
+    "dep:serde_core",
+    "dep:serde",
+]
+
+[lib]
+name = "hashbrown"
+path = "src/lib.rs"
+
+[[test]]
+name = "equivalent_trait"
+path = "tests/equivalent_trait.rs"
+
+[[test]]
+name = "hasher"
+path = "tests/hasher.rs"
+
+[[test]]
+name = "rayon"
+path = "tests/rayon.rs"
+
+[[test]]
+name = "serde"
+path = "tests/serde.rs"
+
+[[test]]
+name = "set"
+path = "tests/set.rs"
+
+[[bench]]
+name = "bench"
+path = "benches/bench.rs"
+
+[[bench]]
+name = "insert_unique_unchecked"
+path = "benches/insert_unique_unchecked.rs"
+
+[[bench]]
+name = "set_ops"
+path = "benches/set_ops.rs"
+
+[[bench]]
+name = "with_capacity"
+path = "benches/with_capacity.rs"
+
+[dependencies.alloc]
+version = "1.0.0"
+optional = true
+package = "rustc-std-workspace-alloc"
+
+[dependencies.allocator-api2]
+version = "0.2.9"
+features = ["alloc"]
+optional = true
+default-features = false
+
+[dependencies.core]
+version = "1.0.0"
+optional = true
+package = "rustc-std-workspace-core"
+
+[dependencies.equivalent]
+version = "1.0"
+optional = true
+default-features = false
+
+[dependencies.foldhash]
+version = "0.2.0"
+optional = true
+default-features = false
+
+[dependencies.rayon]
+version = "1.9.0"
+optional = true
+
+[dependencies.serde_core]
+version = "1.0.221"
+optional = true
+default-features = false
+
+[dev-dependencies.bumpalo]
+version = "3.13.0"
+features = ["allocator-api2"]
+
+[dev-dependencies.fnv]
+version = "1.0.7"
+
+[dev-dependencies.lazy_static]
+version = "1.4"
+
+[dev-dependencies.rand]
+version = "0.9.0"
+features = ["small_rng"]
+
+[dev-dependencies.rayon]
+version = "1.2"
+
+[dev-dependencies.serde_test]
+version = "1.0"
+
+[target."cfg(any())".dependencies.serde]
+version = "1.0.220"
+optional = true
+default-features = false
+
+[target."cfg(unix)".dev-dependencies.libc]
+version = "0.2.155"