Fix #832: --automation-read-root and --automation-write-root confine the MCP server's files

vectorcraft-cli mcp and vectorcraft --control take the flags PhotoCraft
takes. While roots are in force, every path a command reads (open, place,
relink, SVG image links, library and profile loads, plug-ins, preset
imports, font searches) must lie inside the read root and every path it
writes (save, export, package, Export for Screens and for Web, print to
file, captures, library saves) inside the write root. Paths are made
absolute, links are followed (a missing file's nearest folder's), and the
result is compared by whole components, ignoring case on Windows; `..`
below a missing folder, dangling links and device names are refused. A
root left out grants none of its access; without the flags nothing
changes.

The checks live in one place, vectorcraft_engine::file_access, consulted
by the engine's file helpers and the app's file services. The headless
server confines its whole process; the app confines control requests,
the frames carrying input they inject and the background saves they
start, leaving the person at the keyboard and the app's own folders
alone. The screenshot tool now has the backend write its file.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/plugins/src/registry.rs

@@ -95,6 +95,13 @@ pub struct FolderReport {
 /// reported and skipped; the others still load.
 #[cfg(not(target_arch = "wasm32"))]
 pub fn load_folder(dir: &std::path::Path) -> Result<FolderReport> {
+    load_folder_where(dir, |_| Ok(()))
+}
+
+/// [`load_folder`], loading only the files `allow` accepts; the others are reported as failed
+/// with its reason.
+#[cfg(not(target_arch = "wasm32"))]
+pub fn load_folder_where(dir: &std::path::Path, allow: impl Fn(&std::path::Path) -> std::result::Result<(), String>) -> Result<FolderReport> {
     let rd = std::fs::read_dir(dir).map_err(|e| Error::Io(format!("{}: {e}", dir.display())))?;
     let mut files: Vec<std::path::PathBuf> =
         rd.filter_map(|e| e.ok().map(|e| e.path())).filter(|p| p.extension().is_some_and(|x| x.eq_ignore_ascii_case("wasm"))).collect();
@@ -102,6 +109,10 @@ pub fn load_folder(dir: &std::path::Path) -> Result<FolderReport> {
     files.truncate(1000);
     let mut report = FolderReport::default();
     for f in files {
+        if let Err(why) = allow(&f) {
+            report.failed.push((f.display().to_string(), why));
+            continue;
+        }
         match load_file(&f, Limits::default()) {
             Ok(p) => report.loaded.push(install(p).id().to_string()),
             Err(e) => report.failed.push((f.display().to_string(), e.to_string())),