feat(viewer): IPC types, wrappers and Rust module skeleton for the file viewer

Task 0 of the terminal file viewer plan: the shared interfaces that seven
later tasks build against in parallel. Adds ViewerLocation/ViewerTargetState/
ViewerState/ViewerFile/ViewerPoll to types.ts and their invoke() wrappers to
tauri-commands.ts, creates the file_viewer Rust module (mod.rs with
MAX_VIEWER_WINDOWS/VIEWER_LABEL_PREFIX/is_viewer_label, plus placeholder
registry/resolve/poll/write/window submodules), wires it into lib.rs, and
loosens visibility on the file_commands.rs helpers the viewer commands will
reuse (MAX_READ_BYTES, validate_container_path, validate_container_write_path,
FetchedFile, fetch_container_file, require_running, clip_container_text).

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

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

@@ -0,0 +1,36 @@
+//! The terminal file viewer: one OS window per clicked path.
+//!
+//! Every window is a `file-viewer-<n>` label registered in [`registry::ViewerRegistry`];
+//! the commands in `commands/file_viewer_commands.rs` gate on the label and act only on
+//! the caller's own entry, which is why nothing here takes a path from a window.
+
+pub mod poll;
+pub mod registry;
+pub mod resolve;
+pub mod window;
+pub mod write;
+
+/// Spec §3: the 21st click is refused with a toast.
+pub const MAX_VIEWER_WINDOWS: usize = 20;
+pub const VIEWER_LABEL_PREFIX: &str = "file-viewer-";
+
+pub fn is_viewer_label(label: &str) -> bool {
+    label
+        .strip_prefix(VIEWER_LABEL_PREFIX)
+        .is_some_and(|rest| !rest.is_empty() && rest.bytes().all(|b| b.is_ascii_digit()))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn only_numbered_viewer_labels_pass() {
+        assert!(is_viewer_label("file-viewer-1"));
+        assert!(is_viewer_label("file-viewer-20"));
+        assert!(!is_viewer_label("file-viewer-"));
+        assert!(!is_viewer_label("file-viewer-x"));
+        assert!(!is_viewer_label("main"));
+        assert!(!is_viewer_label("browser-view-abc"));
+    }
+}