agent: lumotia — pin rust toolchain + workspace clippy/fmt sweep

rust-toolchain.toml pins to stable 1.94.1 so contributors and CI runners
share the exact rustc / rustfmt / clippy versions. Without the pin, every
machine surfaces a different lint set depending on its local install — six
pre-existing lints showed up on 1.94.1 that 1.93-era HANDOVER reported clean.

Clippy fixes (all pre-existing, not introduced by feature work):

- crates/storage/src/database.rs: std::iter::repeat().take() -> repeat_n()
- crates/llm/src/lib.rs (docs): "+ frontends" was parsed as a markdown bullet
  continuation by rustdoc, breaking doc-lazy-continuation. Reworded to "and".
- crates/llm/src/lib.rs (loop): while-let-on-iterator -> for-loop.
- src-tauri/src/commands/security.rs: .iter().any(|a| *a == x) -> .contains(&x).
- src-tauri/src/lib.rs: io::Error::new(Other, e) -> io::Error::other(e).
- src-tauri/src/tauri_app_data_migration.rs: drop function-tail `return`s
  inside cfg blocks; each platform's block now ends with a tail expression.

cargo fmt sweep across the workspace. Mechanical layout-only changes;
no semantics affected.

Workspace gates after this commit:
- cargo fmt --check: clean
- cargo clippy --workspace --all-targets -- -D warnings: clean
- cargo test --workspace: 405/0 (will become 409/0 with Phase A.1+A.2)

src-tauri/src/tauri_app_data_migration.rs

@@ -41,16 +41,10 @@ pub enum AppDataMigrationStatus {
     /// after the first successful migration (we preserve legacy as a
     /// backup), so the warning is informational rather than an
     /// indication of trouble.
-    BothExistLegacyPreserved {
-        old: PathBuf,
-        new: PathBuf,
-    },
+    BothExistLegacyPreserved { old: PathBuf, new: PathBuf },
     /// Migration succeeded: legacy copied to new path via atomic
     /// staging rename, legacy preserved as a backup.
-    Migrated {
-        old: PathBuf,
-        new: PathBuf,
-    },
+    Migrated { old: PathBuf, new: PathBuf },
 }
 
 /// Resolve the OLD Tauri `app_data_dir` from platform conventions. The
@@ -67,6 +61,10 @@ pub fn legacy_tauri_app_data_dir() -> Option<PathBuf> {
 }
 
 fn legacy_tauri_app_data_dir_for(identifier: &str) -> Option<PathBuf> {
+    // Exactly one of the four cfg blocks below is present per target
+    // compile. Each is a tail expression that becomes the function's
+    // return value. Avoiding explicit `return` keeps clippy's
+    // needless_return lint happy on every platform.
     #[cfg(target_os = "linux")]
     {
         // XDG_DATA_HOME wins when set and non-empty, per the XDG Base
@@ -79,29 +77,29 @@ fn legacy_tauri_app_data_dir_for(identifier: &str) -> Option<PathBuf> {
             }
         }
         let home = std::env::var("HOME").ok().filter(|s| !s.is_empty())?;
-        return Some(
+        Some(
             PathBuf::from(home)
                 .join(".local")
                 .join("share")
                 .join(identifier),
-        );
+        )
     }
 
     #[cfg(target_os = "macos")]
     {
         let home = std::env::var("HOME").ok().filter(|s| !s.is_empty())?;
-        return Some(
+        Some(
             PathBuf::from(home)
                 .join("Library")
                 .join("Application Support")
                 .join(identifier),
-        );
+        )
     }
 
     #[cfg(target_os = "windows")]
     {
         let appdata = std::env::var("APPDATA").ok().filter(|s| !s.is_empty())?;
-        return Some(PathBuf::from(appdata).join(identifier));
+        Some(PathBuf::from(appdata).join(identifier))
     }
 
     #[cfg(not(any(target_os = "linux", target_os = "macos", target_os = "windows")))]
@@ -123,10 +121,7 @@ fn legacy_tauri_app_data_dir_for(identifier: &str) -> Option<PathBuf> {
 /// `paths.rs` migration already covers the user's transcripts and
 /// models, so even a total-failure here only loses webview-keyed state
 /// (preferences, session storage, plugin geometry).
-pub fn migrate_tauri_app_data_dir_with_paths(
-    old: &Path,
-    new: &Path,
-) -> AppDataMigrationStatus {
+pub fn migrate_tauri_app_data_dir_with_paths(old: &Path, new: &Path) -> AppDataMigrationStatus {
     let old_exists = old.exists();
     let new_exists = new.exists();
 
@@ -265,7 +260,10 @@ mod tests {
         assert!(leveldb.exists());
 
         // Staging directory is cleaned up.
-        assert!(!tmp.path().join("consulting.corbel.lumotia.migrating").exists());
+        assert!(!tmp
+            .path()
+            .join("consulting.corbel.lumotia.migrating")
+            .exists());
     }
 
     #[test]
@@ -329,6 +327,9 @@ mod tests {
             AppDataMigrationStatus::BothExistLegacyPreserved { .. }
         ));
 
-        assert_eq!(fs::read(new.join("file.txt")).unwrap(), b"v2-edited-by-user");
+        assert_eq!(
+            fs::read(new.join("file.txt")).unwrap(),
+            b"v2-edited-by-user"
+        );
     }
 }