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/indexmap/Cargo.toml

@@ -0,0 +1,167 @@
+# 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 = "1.82"
+name = "indexmap"
+version = "2.13.0"
+build = false
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "A hash table with consistent order and fast iteration."
+documentation = "https://docs.rs/indexmap/"
+readme = "README.md"
+keywords = [
+    "hashmap",
+    "no_std",
+]
+categories = [
+    "data-structures",
+    "no-std",
+]
+license = "Apache-2.0 OR MIT"
+repository = "https://github.com/indexmap-rs/indexmap"
+
+[package.metadata.release]
+allow-branch = ["main"]
+sign-tag = true
+tag-name = "{{version}}"
+
+[package.metadata.docs.rs]
+features = [
+    "arbitrary",
+    "quickcheck",
+    "serde",
+    "borsh",
+    "rayon",
+    "sval",
+]
+rustdoc-args = [
+    "--cfg",
+    "docsrs",
+]
+
+[features]
+default = ["std"]
+serde = [
+    "dep:serde_core",
+    "dep:serde",
+]
+std = []
+test_debug = []
+
+[lib]
+name = "indexmap"
+path = "src/lib.rs"
+bench = false
+
+[[test]]
+name = "equivalent_trait"
+path = "tests/equivalent_trait.rs"
+
+[[test]]
+name = "macros_full_path"
+path = "tests/macros_full_path.rs"
+
+[[test]]
+name = "quick"
+path = "tests/quick.rs"
+
+[[test]]
+name = "tests"
+path = "tests/tests.rs"
+
+[[bench]]
+name = "bench"
+path = "benches/bench.rs"
+
+[[bench]]
+name = "faststring"
+path = "benches/faststring.rs"
+
+[dependencies.arbitrary]
+version = "1.0"
+optional = true
+default-features = false
+
+[dependencies.borsh]
+version = "1.2"
+optional = true
+default-features = false
+
+[dependencies.equivalent]
+version = "1.0"
+default-features = false
+
+[dependencies.hashbrown]
+version = "0.16.1"
+default-features = false
+
+[dependencies.quickcheck]
+version = "1.0"
+optional = true
+default-features = false
+
+[dependencies.rayon]
+version = "1.9"
+optional = true
+
+[dependencies.serde_core]
+version = "1.0.220"
+optional = true
+default-features = false
+
+[dependencies.sval]
+version = "2"
+optional = true
+default-features = false
+
+[dev-dependencies.fastrand]
+version = "2"
+default-features = false
+
+[dev-dependencies.fnv]
+version = "1.0"
+
+[dev-dependencies.itertools]
+version = "0.14"
+
+[dev-dependencies.quickcheck]
+version = "1.0"
+default-features = false
+
+[dev-dependencies.serde]
+version = "1.0"
+features = ["derive"]
+default-features = false
+
+[target."cfg(any())".dependencies.serde]
+version = "1.0.220"
+optional = true
+default-features = false
+
+[lints.clippy]
+style = "allow"
+
+[lints.rust]
+private-bounds = "deny"
+private-interfaces = "deny"
+rust-2018-idioms = "warn"
+unnameable-types = "deny"
+unreachable-pub = "deny"
+unsafe-code = "deny"
+
+[profile.bench]
+debug = 2