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/pulldown-cmark/Cargo.toml

@@ -0,0 +1,118 @@
+# 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 = "1.71.1"
+name = "pulldown-cmark"
+version = "0.11.3"
+authors = [
+    "Raph Levien <raph.levien@gmail.com>",
+    "Marcus Klaas de Vries <mail@marcusklaas.nl>",
+]
+build = "build.rs"
+exclude = [
+    "/third_party/**/*",
+    "/tools/**/*",
+    "/specs/**/*",
+    "/azure-pipelines.yml",
+]
+description = "A pull parser for CommonMark"
+readme = "README.md"
+keywords = [
+    "markdown",
+    "commonmark",
+]
+categories = ["text-processing"]
+license = "MIT"
+repository = "https://github.com/raphlinus/pulldown-cmark"
+
+[[bin]]
+name = "pulldown-cmark"
+doc = false
+required-features = ["getopts"]
+
+[[example]]
+name = "event-filter"
+doc-scrape-examples = true
+required-features = ["html"]
+
+[[example]]
+name = "footnote-rewrite"
+doc-scrape-examples = true
+required-features = ["html"]
+
+[[example]]
+name = "parser-map-event-print"
+doc-scrape-examples = true
+required-features = ["html"]
+
+[[example]]
+name = "parser-map-tag-print"
+doc-scrape-examples = true
+required-features = ["html"]
+
+[[example]]
+name = "string-to-string"
+doc-scrape-examples = true
+required-features = ["html"]
+
+[[example]]
+name = "broken-link-callbacks"
+doc-scrape-examples = true
+required-features = ["html"]
+
+[dependencies.bitflags]
+version = "2"
+
+[dependencies.getopts]
+version = "0.2"
+optional = true
+
+[dependencies.memchr]
+version = "2.5"
+
+[dependencies.pulldown-cmark-escape]
+version = "0.11"
+optional = true
+
+[dependencies.serde]
+version = "1.0"
+features = ["derive"]
+optional = true
+
+[dependencies.unicase]
+version = "2.6"
+
+[dev-dependencies.bincode]
+version = "1.3.1"
+
+[dev-dependencies.lazy_static]
+version = "1.4"
+
+[dev-dependencies.regex]
+version = "1.6"
+
+[dev-dependencies.serde_json]
+version = "1.0.61"
+
+[features]
+default = [
+    "getopts",
+    "html",
+]
+gen-tests = []
+html = ["pulldown-cmark-escape"]
+simd = ["pulldown-cmark-escape?/simd"]
+
+[lints.rust.unexpected_cfgs]
+level = "warn"
+priority = 0