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/embassy-executor/build.rs

@@ -0,0 +1,99 @@
+use std::collections::HashMap;
+use std::fmt::Write;
+use std::path::PathBuf;
+use std::{env, fs};
+
+#[path = "./build_common.rs"]
+mod common;
+
+static CONFIGS: &[(&str, usize)] = &[
+    // BEGIN AUTOGENERATED CONFIG FEATURES
+    // Generated by gen_config.py. DO NOT EDIT.
+    ("TASK_ARENA_SIZE", 4096),
+    // END AUTOGENERATED CONFIG FEATURES
+];
+
+struct ConfigState {
+    value: usize,
+    seen_feature: bool,
+    seen_env: bool,
+}
+
+fn main() {
+    let crate_name = env::var("CARGO_PKG_NAME")
+        .unwrap()
+        .to_ascii_uppercase()
+        .replace('-', "_");
+
+    // only rebuild if build.rs changed. Otherwise Cargo will rebuild if any
+    // other file changed.
+    println!("cargo:rerun-if-changed=build.rs");
+
+    // Rebuild if config envvar changed.
+    for (name, _) in CONFIGS {
+        println!("cargo:rerun-if-env-changed={crate_name}_{name}");
+    }
+
+    let mut configs = HashMap::new();
+    for (name, default) in CONFIGS {
+        configs.insert(
+            *name,
+            ConfigState {
+                value: *default,
+                seen_env: false,
+                seen_feature: false,
+            },
+        );
+    }
+
+    let prefix = format!("{crate_name}_");
+    for (var, value) in env::vars() {
+        if let Some(name) = var.strip_prefix(&prefix) {
+            let Some(cfg) = configs.get_mut(name) else {
+                panic!("Unknown env var {name}")
+            };
+
+            let Ok(value) = value.parse::<usize>() else {
+                panic!("Invalid value for env var {name}: {value}")
+            };
+
+            cfg.value = value;
+            cfg.seen_env = true;
+        }
+
+        if let Some(feature) = var.strip_prefix("CARGO_FEATURE_") {
+            if let Some(i) = feature.rfind('_') {
+                let name = &feature[..i];
+                let value = &feature[i + 1..];
+                if let Some(cfg) = configs.get_mut(name) {
+                    let Ok(value) = value.parse::<usize>() else {
+                        panic!("Invalid value for feature {name}: {value}")
+                    };
+
+                    // envvars take priority.
+                    if !cfg.seen_env {
+                        if cfg.seen_feature {
+                            panic!("multiple values set for feature {}: {} and {}", name, cfg.value, value);
+                        }
+
+                        cfg.value = value;
+                        cfg.seen_feature = true;
+                    }
+                }
+            }
+        }
+    }
+
+    let mut data = String::new();
+
+    for (name, cfg) in &configs {
+        writeln!(&mut data, "pub const {}: usize = {};", name, cfg.value).unwrap();
+    }
+
+    let out_dir = PathBuf::from(env::var_os("OUT_DIR").unwrap());
+    let out_file = out_dir.join("config.rs").to_string_lossy().to_string();
+    fs::write(out_file, data).unwrap();
+
+    let mut rustc_cfgs = common::CfgSet::new();
+    common::set_target_cfgs(&mut rustc_cfgs);
+}