feat(open): open drawings in the running editor

Double-clicking a drawing started a whole second copy of the app. Now it
lands as a tab in the editor already open, and that window comes forward.

The election is the bind itself: every GUI launch tries a deterministic
per-user loopback port, and the OS grants it to exactly one process and
releases it on any death, SIGKILL included. Nothing to reap, no PID to
probe, no stale lock after a crash. The winner serves the port from an
iced subscription; the loser checks it is talking to a matching editor,
hands the paths over and exits. Every surprise — no answer, a stranger on
the port, a timeout — boots a normal window, i.e. the old behaviour: the
feature can degrade, but it cannot lose the file.

The gate in main is positional, after every headless mode has returned.
The plugin runner is this same binary re-spawning itself, so a flag list
would rot the first time a mode is added; a position cannot.

Handing a drawing over that is already open switches to its tab instead
of loading a second copy, comparing resolved paths so a symlink or a
`..` hop is still recognised as the same file.

`opening` is a single slot that a second open would overwrite, and
on_file_opened drops any result arriving once it is clear — so drawings
handed over at once would silently vanish. They queue in pending_opens,
drained from every path that clears the slot, error and cancel included.

Exec becomes %F so one launch takes the whole selection; the CLI takes a
list and boot fans it out through the same door. --new-instance forces a
separate process; --read-only / --script / --new always get their own.

Not reused: the automation server. It dispatches arbitrary commands and
writes arbitrary paths, and its open replaces the active tab's document
in place — publishing that on a port every local process can reach would
be unforced privilege escalation. Only its JSON-lines framing is shared.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/main.rs

@@ -100,6 +100,29 @@ fn main() -> iced::Result {
             std::process::exit(code);
         }
 
+        // Single instance: a double-clicked drawing belongs as a tab in the
+        // editor that is already open, not in a second copy of the app.
+        //
+        // The gate is POSITIONAL, and that is the point: every headless mode
+        // has already returned above — the plugin runner, which is this same
+        // binary re-spawning itself, most of all. A flag list here would rot
+        // the first time a mode is added; a position cannot.
+        if !args.new_instance {
+            if let io::single_instance::Claim::Existing(stream) = io::single_instance::claim() {
+                // Only bare files forward. `--read-only` / `--script` / `--new`
+                // configure the whole editor rather than a tab, so they always
+                // get a process of their own.
+                let plain_open =
+                    !args.read_only && args.script.is_none() && !args.new && !args.files.is_empty();
+                if plain_open && io::single_instance::handoff(stream, &args.files) {
+                    return Ok(());
+                }
+                // Nothing to forward, or the far end never acknowledged: fall
+                // through and boot our own window. We hold no listener, so the
+                // editor that owns the port keeps serving.
+            }
+        }
+
         // GUI: stash the startup config for `app::boot` to pick up.
         let script_lines = args
             .script
@@ -118,7 +141,7 @@ fn main() -> iced::Result {
             })
             .unwrap_or_default();
         let _ = cli::GUI_CONFIG.set(cli::GuiConfig {
-            file: if args.new { None } else { args.file },
+            files: if args.new { Vec::new() } else { args.files },
             new: args.new,
             read_only: args.read_only,
             script_lines,