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/postcard/Cargo.toml

@@ -0,0 +1,136 @@
+# 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 = "postcard"
+version = "1.1.3"
+authors = ["James Munns <james@onevariable.com>"]
+build = false
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "A no_std + serde compatible message library for Rust"
+documentation = "https://docs.rs/postcard/"
+readme = "README.md"
+keywords = [
+    "serde",
+    "cobs",
+    "framing",
+]
+categories = [
+    "embedded",
+    "no-std",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/jamesmunns/postcard"
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = [
+    "--cfg",
+    "docsrs",
+]
+
+[features]
+alloc = [
+    "serde/alloc",
+    "embedded-io-04?/alloc",
+    "embedded-io-06?/alloc",
+]
+core-num-saturating = []
+crc = ["dep:crc"]
+default = ["heapless-cas"]
+defmt = ["dep:defmt"]
+embedded-io = ["dep:embedded-io-04"]
+embedded-io-04 = ["dep:embedded-io-04"]
+embedded-io-06 = ["dep:embedded-io-06"]
+experimental-derive = ["postcard-derive"]
+heapless = ["dep:heapless"]
+heapless-cas = [
+    "heapless",
+    "dep:heapless",
+    "heapless/cas",
+]
+nalgebra-v0_33 = ["nalgebra_v0_33"]
+paste = []
+postcard-derive = ["dep:postcard-derive"]
+use-crc = ["crc"]
+use-defmt = ["defmt"]
+use-std = [
+    "serde/std",
+    "alloc",
+]
+
+[lib]
+name = "postcard"
+path = "src/lib.rs"
+
+[[test]]
+name = "accumulator"
+path = "tests/accumulator.rs"
+
+[[test]]
+name = "crc"
+path = "tests/crc.rs"
+
+[[test]]
+name = "loopback"
+path = "tests/loopback.rs"
+
+[[test]]
+name = "max_size"
+path = "tests/max_size.rs"
+
+[dependencies.cobs]
+version = "0.3.0"
+default-features = false
+
+[dependencies.crc]
+version = "3.0.1"
+optional = true
+
+[dependencies.defmt]
+version = "1"
+optional = true
+
+[dependencies.embedded-io-04]
+version = "0.4"
+optional = true
+package = "embedded-io"
+
+[dependencies.embedded-io-06]
+version = "0.6"
+optional = true
+package = "embedded-io"
+
+[dependencies.heapless]
+version = "0.7.0"
+features = ["serde"]
+optional = true
+default-features = false
+
+[dependencies.nalgebra_v0_33]
+version = "0.33.0"
+optional = true
+default-features = false
+package = "nalgebra"
+
+[dependencies.postcard-derive]
+version = "0.2.0"
+optional = true
+
+[dependencies.serde]
+version = "1.0.100"
+features = ["derive"]
+default-features = false