redbear-btctl: convert StubBackend::from_env panic to Result propagation

The single production-code panic in the redbear-* codebase was in
StubBackend::from_env() (backend.rs:192). It panicked when the
REDBEAR_BTCTL_STUB_ADAPTERS env var contained an invalid Bluetooth adapter
name.

Previously:
  - from_env() returned Self (infallible)
  - build_backend() returned Box<dyn Backend>
  - main() returned anyhow::Result<()>

After:
  - from_env() renamed to try_from_env() -> Result<Self, String>
  - build_backend() returns anyhow::Result<Box<dyn Backend>>
  - main() call sites use '?' to propagate errors
  - Adapter name validation now returns Err instead of crashing the daemon

The panic was the ONLY production-code panic across all 5 redbear-*
programs that this phase was scoped for. The other 13 panics are in
test code (#[cfg(test)] modules), which is idiomatic Rust testing
practice and does not need modification.

cargo check --manifest-path source/Cargo.toml passes cleanly.

local/recipes/system/redbear-btctl/source/src/backend.rs

@@ -176,7 +176,7 @@ pub struct StubBackend {
 }
 
 impl StubBackend {
-    pub fn from_env() -> Self {
+    pub fn try_from_env() -> Result<Self, String> {
         let adapters = parse_list(
             env::var("REDBEAR_BTCTL_STUB_ADAPTERS").ok().as_deref(),
             &["hci0"],
@@ -188,12 +188,14 @@ impl StubBackend {
             &[],
         );
         for adapter in &adapters {
-            if validate_adapter_name(adapter).is_err() {
-                panic!("invalid Bluetooth adapter name in REDBEAR_BTCTL_STUB_ADAPTERS: {adapter}");
+            if let Err(e) = validate_adapter_name(adapter) {
+                return Err(format!(
+                    "invalid Bluetooth adapter name in REDBEAR_BTCTL_STUB_ADAPTERS: {adapter}: {e}"
+                ));
             }
         }
 
-        Self {
+        Ok(Self {
             runtime_state: adapters
                 .iter()
                 .cloned()
@@ -212,7 +214,7 @@ impl StubBackend {
                 .map(PathBuf::from)
                 .unwrap_or_else(|| PathBuf::from("/var/run/redbear-btusb/status")),
             bond_store: BondStore::from_env(),
-        }
+        })
     }
 
     #[cfg(test)]

local/recipes/system/redbear-btctl/source/src/main.rs

@@ -73,12 +73,14 @@ fn notify_scheme_ready(notify_fd: Option<RawFd>, socket: &Socket, scheme: &mut B
     }
 }
 
-fn build_backend() -> Box<dyn Backend> {
+fn build_backend() -> anyhow::Result<Box<dyn Backend>> {
     let backend_type = env::var("REDBEAR_BTCTL_BACKEND")
         .unwrap_or_else(|_| "stub".to_string());
     match backend_type.as_str() {
-        "hci" => Box::new(hci_backend::HciBackend::from_env()),
-        _ => Box::new(StubBackend::from_env()),
+        "hci" => Ok(Box::new(hci_backend::HciBackend::from_env())),
+        _ => Ok(Box::new(StubBackend::try_from_env().map_err(|e| {
+            anyhow::anyhow!("{e}")
+        })?)),
     }
 }
 
@@ -424,7 +426,7 @@ fn main() -> anyhow::Result<()> {
     init_logging(log_level);
 
     let args = env::args().skip(1).collect::<Vec<_>>();
-    let mut backend = build_backend();
+    let mut backend = build_backend()?;
 
     match execute(&args, backend.as_mut()) {
         Ok(Some(output)) => {
@@ -452,7 +454,7 @@ fn main() -> anyhow::Result<()> {
                 process::exit(1);
             }
         };
-        let mut scheme = BtCtlScheme::new(build_backend());
+        let mut scheme = BtCtlScheme::new(build_backend()?);
         let mut state = redox_scheme::scheme::SchemeState::new();
 
         notify_scheme_ready(notify_fd, &socket, &mut scheme);