Fix #34: Ctrl/Cmd+Shift+V pastes in place

Paste in Place is Cmd+Shift+V, but egui reports every Cmd+V chord as a
Paste event (or, for what isn't text, only the key's release), never as
the key, and the handler ran Paste for every Paste event. A paste now runs
the paste command whose V chord is held (Paste in Place, Paste on All
Artboards, Paste without Formatting; user overrides included), Paste for
the Paste key and Shift+Insert. Envelope Distort > Edit Contents also had
Cmd+Shift+V; it loses the duplicate, and a test checks that no two
commands, tools or panels share a default shortcut.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/live.rs

@@ -142,7 +142,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "object.envelope.editContents",
             "Edit Contents",
             ["Object", "Envelope Distort"],
-            Some("Cmd+Shift+V"),
+            None,
             "{editing?: bool} toggle Edit Contents / Edit Envelope → {editing}",
             has_envelope,
             env_edit_contents