feat(editor): add wireCollab extension point + typed mutation events

[lassediercks]

Adds a small extension point that downstream consumers can use to mirror the editor's in-memory graph into an external store without copying code or shadowing internal modules.

Two pieces:

1. Extended wireMutations (src/editor/mutations.ts) — onMutate callback fires alongside scheduleSave for every mutation. The event carries the kind ("box" | "edge" | "text" | "line" | "stroke" | "currentMap" | "doc") and the current map path so a consumer can scope a diff to the affected slice.

   getMapPath is also new and optional (defaults to "/"). All existing wireMutations callers continue to compile and run unchanged.

2. New src/editor/collab-api.ts — exports whenCollabReady(cb) and a CollabHandle interface:

   interface CollabHandle {
   snapshot(): FlowgoGraph;
   applyRemotePatch(fn: (g: FlowgoGraph) => void): void;
   onLocalMutation(cb: (e: MutationEvent) => void): () => void;
   }

   Plugins call whenCollabReady from a separate bundle; the editor's main.ts calls exposeCollabHandle at the end of bootstrap with an impl that snapshots the live graph (structuredClone), applies remote patches in place + re-renders, and fans onMutate events out to subscribers.

   FlowgoGraph + FlowgoBox/Edge/Text/Line/Stroke types mirror the pkg/graph.Graph shape verbatim so a plugin can pass snapshots to a Go/Rust sidecar via JSON without translation.

./collab added to the package.json exports map so consumers can import "@flowgo/editor/collab".

Bundle size: 103.82 KB (+414 bytes vs main, almost all from the type-only declarations that get erased at build time).

All 120 vitest tests pass; tsc --noEmit clean; vite build clean. Zero behaviour change for CLI / single-user consumers — the new file ships in every bundle but does nothing until a downstream plugin calls whenCollabReady.