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/riscv/Cargo.toml

@@ -0,0 +1,73 @@
+# 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 = "1.61"
+name = "riscv"
+version = "0.12.1"
+authors = ["The RISC-V Team <risc-v@teams.rust-embedded.org>"]
+build = "build.rs"
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "Low level access to RISC-V processors"
+documentation = "https://docs.rs/riscv"
+readme = "README.md"
+keywords = [
+    "riscv",
+    "register",
+    "peripheral",
+]
+categories = [
+    "embedded",
+    "hardware-support",
+    "no-std",
+]
+license = "ISC"
+repository = "https://github.com/rust-embedded/riscv"
+
+[package.metadata.docs.rs]
+all-features = true
+default-target = "riscv64imac-unknown-none-elf"
+targets = [
+    "riscv32i-unknown-none-elf",
+    "riscv32imc-unknown-none-elf",
+    "riscv32imac-unknown-none-elf",
+    "riscv64imac-unknown-none-elf",
+    "riscv64gc-unknown-none-elf",
+]
+
+[lib]
+name = "riscv"
+path = "src/lib.rs"
+
+[dependencies.critical-section]
+version = "1.2.0"
+
+[dependencies.embedded-hal]
+version = "1.0.0"
+
+[dependencies.paste]
+version = "1.0.15"
+
+[dependencies.riscv-macros]
+version = "0.1.0"
+optional = true
+
+[dependencies.riscv-pac]
+version = "0.2.0"
+
+[features]
+critical-section-single-hart = ["critical-section/restore-state-bool"]
+default = ["riscv-macros"]
+s-mode = []