docs: terminal file viewer structure and capability census

Add CLAUDE.md structure notes for the viewer/ frontend and file_viewer/
backend, plus a Key Conventions reminder that a new local window needs its
own capability file and lib.rs's on_window_event guard. Extend default.json's
threat-model census to name file-viewer.json and its allow-destroy grant.

Tighten the capability test in file_viewer/mod.rs from a substring check to
a parsed serde_json assertion of the exact windows list and permission set
for both capability files, per Task 12's controller ruling.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

app/src-tauri/src/file_viewer/mod.rs

@@ -45,7 +45,53 @@ mod tests {
         assert!(!html.contains("<style"), "an inline <style> makes Tauri add a style nonce, which disables 'unsafe-inline' and breaks CodeMirror");
         let vite = std::fs::read_to_string(app_dir.join("vite.config.ts")).expect("vite.config.ts");
         assert!(vite.contains("viewer.html"), "vite.config.ts must list viewer.html in build.rollupOptions.input");
-        let cap = std::fs::read_to_string(app_dir.join("src-tauri/capabilities/file-viewer.json")).expect("capability");
-        assert!(cap.contains("\"file-viewer-*\"") && cap.contains("core:window:allow-destroy"));
+    }
+
+    #[derive(serde::Deserialize)]
+    struct Capability {
+        windows: Vec<String>,
+        permissions: Vec<String>,
+    }
+
+    /// Task 12: a substring check on the capability JSON (the form this test used to take)
+    /// only proves a permission string appears *somewhere* in the file — it would not catch
+    /// `windows` widened past `file-viewer-*`, nor an extra grant slipped in beside the ones
+    /// this window actually needs. Parse both capability files and pin `windows`/`permissions`
+    /// exactly, so a later widening of either file is a failing test, not a silent threat-model
+    /// drift — this file *is* the reviewed threat model of record (see its own description).
+    #[test]
+    fn the_viewer_capability_grants_exactly_the_reviewed_windows_and_permissions() {
+        let app_dir = std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("..");
+        let raw = std::fs::read_to_string(app_dir.join("src-tauri/capabilities/file-viewer.json"))
+            .expect("capabilities/file-viewer.json");
+        let cap: Capability = serde_json::from_str(&raw).expect("file-viewer.json must be valid JSON");
+
+        assert_eq!(cap.windows, vec!["file-viewer-*"]);
+
+        let mut permissions = cap.permissions;
+        permissions.sort();
+        assert_eq!(
+            permissions,
+            vec![
+                "core:event:allow-listen",
+                "core:event:allow-unlisten",
+                "core:webview:allow-internal-toggle-devtools",
+                "core:window:allow-destroy",
+            ]
+        );
+    }
+
+    /// The main window's capability file must stay scoped to `main` — a `windows` list that
+    /// grew to include `file-viewer-*` would hand every viewer window the dialog/store surface
+    /// `default.json` grants `main`, which is a much larger IPC surface than the one
+    /// `file-viewer.json` was deliberately kept small.
+    #[test]
+    fn the_default_capability_is_scoped_to_the_main_window_only() {
+        let app_dir = std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("..");
+        let raw = std::fs::read_to_string(app_dir.join("src-tauri/capabilities/default.json"))
+            .expect("capabilities/default.json");
+        let cap: Capability = serde_json::from_str(&raw).expect("default.json must be valid JSON");
+
+        assert_eq!(cap.windows, vec!["main"]);
     }
 }