feat(phase9): write_text_file_cmd

Thin UTF-8 writer used by the new save-dialog path. Caller owns path
safety; the source path is always OS-dialog-provided. Two unit tests:
roundtrips a small UTF-8 string with non-ASCII chars and asserts a
nonexistent parent path returns an actionable error.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/src/commands/fs.rs

@@ -0,0 +1,44 @@
+// Phase 9. Thin filesystem write command for the save-dialog path.
+//
+// Path safety: the caller is expected to obtain `path` via the OS save
+// dialog (tauri-plugin-dialog). We do not validate traversal, extension,
+// or parent-dir existence beyond what the OS filesystem reports — the
+// dialog already constrains the user's choice to what they can write to.
+
+/// Write UTF-8 text to a user-chosen path. Errors propagate the OS
+/// message verbatim wrapped with the path so the toast on the frontend
+/// is actionable ("Failed to write …: Permission denied" rather than a
+/// bare error code).
+#[tauri::command]
+pub async fn write_text_file_cmd(path: String, contents: String) -> Result<(), String> {
+    tokio::fs::write(&path, contents)
+        .await
+        .map_err(|e| format!("Failed to write {path}: {e}"))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn write_text_file_roundtrips_utf8() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join("out.md").display().to_string();
+        write_text_file_cmd(path.clone(), "hello\nwørld\n".into())
+            .await
+            .expect("write");
+
+        let round = tokio::fs::read_to_string(&path).await.expect("read");
+        assert_eq!(round, "hello\nwørld\n");
+    }
+
+    #[tokio::test]
+    async fn write_text_file_errors_on_bad_parent() {
+        let result = write_text_file_cmd(
+            "/definitely-not-a-real-path-kon-phase9/out.md".into(),
+            "x".into(),
+        )
+        .await;
+        assert!(result.is_err(), "expected error for nonexistent parent");
+    }
+}

src-tauri/src/commands/mod.rs

@@ -2,6 +2,7 @@ pub mod audio;
 pub mod clipboard;
 pub mod diagnostics;
 pub mod feedback;
+pub mod fs;
 pub mod hardware;
 pub mod hotkey;
 pub mod intentions;