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/heapless/Cargo.toml

@@ -0,0 +1,91 @@
+# 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 = "heapless"
+version = "0.8.0"
+authors = [
+    "Jorge Aparicio <jorge@japaric.io>",
+    "Per Lindgren <per.lindgren@ltu.se>",
+    "Emil Fresk <emil.fresk@gmail.com>",
+]
+description = "`static` friendly data structures that don't require dynamic memory allocation"
+documentation = "https://docs.rs/heapless"
+readme = "README.md"
+keywords = [
+    "static",
+    "no-heap",
+]
+categories = [
+    "data-structures",
+    "no-std",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/rust-embedded/heapless"
+
+[package.metadata.docs.rs]
+features = [
+    "ufmt",
+    "serde",
+    "defmt-03",
+    "mpmc_large",
+    "portable-atomic-critical-section",
+]
+rustdoc-args = [
+    "--cfg",
+    "docsrs",
+]
+targets = ["i686-unknown-linux-gnu"]
+
+[dependencies.defmt]
+version = ">=0.2.0,<0.4"
+optional = true
+
+[dependencies.hash32]
+version = "0.3.0"
+
+[dependencies.portable-atomic]
+version = "1.0"
+optional = true
+
+[dependencies.serde]
+version = "1"
+optional = true
+default-features = false
+
+[dependencies.stable_deref_trait]
+version = "1"
+default-features = false
+
+[dependencies.ufmt-write]
+version = "0.1"
+optional = true
+
+[dev-dependencies.ufmt]
+version = "0.2"
+
+[features]
+defmt-03 = ["dep:defmt"]
+mpmc_large = []
+portable-atomic = ["dep:portable-atomic"]
+portable-atomic-critical-section = [
+    "dep:portable-atomic",
+    "portable-atomic",
+    "portable-atomic?/critical-section",
+]
+portable-atomic-unsafe-assume-single-core = [
+    "dep:portable-atomic",
+    "portable-atomic",
+    "portable-atomic?/unsafe-assume-single-core",
+]
+serde = ["dep:serde"]
+ufmt = ["dep:ufmt-write"]