Web files: designs live in the browser (OPFS), files come in by picker or drop and go out as downloads

- solvecraft_io::vfs: the engine's file access (disk natively; in the browser an in-memory set of
  files the host loads and stores). File commands and write_atomic go through it.
- Web host (apps/solvecraft-web/src/files.rs): stored designs are read from OPFS at start
  (storage.persist() asked for) and written back when a command writes one; Save/Save As keep
  designs in the browser, exports (STEP, STL, 3MF, OBJ, ...) download as files.
- File > Open shows "Files in this browser": open, insert, rename, download, delete, plus open or
  insert from the computer (showOpenFilePicker where there is one, an <input type=file>
  otherwise). Files dropped on the page are inserted or opened.
- Unsaved work is autosaved to OPFS and offered again after a reload or crash.
- Start page recents come from the stored designs, with thumbnails. In the browser the samples
  are built when opened (a picture blocks the page there, and a kernel panic can't be caught).
- Pages without OPFS (http on a network address, old browsers) keep designs in local storage.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/engine/src/cmd/parameters.rs

@@ -219,7 +219,7 @@ fn export(s: &mut Session, p: &Value) -> Result<Value> {
         other => return Err(bad(cmd, format!("unknown format `{other}` (csv or json)"))),
     };
     if let Some(path) = path {
-        std::fs::write(path, &text).map_err(|e| EngineError::Other(format!("{path}: {e}")))?;
+        solvecraft_io::vfs::write(path, text.as_bytes()).map_err(|e| EngineError::Other(format!("{path}: {e}")))?;
     }
     Ok(json!({"format": format, "path": path, "text": text}))
 }
@@ -229,11 +229,11 @@ fn import(s: &mut Session, p: &Value) -> Result<Value> {
     let text = match (str_(p, "text"), str_(p, "path")) {
         (Some(t), _) => t.to_string(),
         (None, Some(path)) => {
-            let meta = std::fs::metadata(path).map_err(|e| EngineError::Other(format!("{path}: {e}")))?;
-            if meta.len() > 16 << 20 {
+            let meta = solvecraft_io::vfs::len(path).map_err(|e| EngineError::Other(format!("{path}: {e}")))?;
+            if meta > 16 << 20 {
                 return Err(bad(cmd, "file too large"));
             }
-            std::fs::read_to_string(path).map_err(|e| EngineError::Other(format!("{path}: {e}")))?
+            solvecraft_io::vfs::read_to_string(path).map_err(|e| EngineError::Other(format!("{path}: {e}")))?
         }
         _ => return Err(bad(cmd, "give `text` or `path`")),
     };