fix(mvc.Collection): id lookups can return the wrong model when a user id collides with a cid

[kumilingus]

Description

mvc.Collection stored models in a single _byId map under both their model id and their internal client id (cid). cids are generated as c<counter> with a counter global to the JS realm that never resets — so a user-supplied id matching /^c\d+$/ (e.g. "c856") is clobbered as soon as the session has created that many models and some other model's auto-generated cid collides with it.

Two silent failure modes:

• graph.getCell('c856') returns the wrong cell — and everything downstream of it: link end resolution, findViewByModel, etc.
• worse, set()/add() dedup runs through get(), so a genuinely new model whose cid collides with an existing model's id is silently merged away as a duplicate and never added.

Both are timing/volume dependent — a graph loads fine on a fresh page and breaks after enough models have been created in the session — which makes them look like heisenbugs.

Observed in the wild: demos/libavoid-standalone-link-routing example-2.json had an element with id "c856"; after loading a second large diagram, a link's cid collided, @joint/router-avoid's validateEnds() resolved the link's end to another link and permanently fallback-routed it.

Fix

Ids and cids now live in separate maps (_byId / _byCid):

• get() checks real ids first — a real id always wins the string lookup;
• cid string lookup (collection.get('c12'), Backbone compatibility) still works whenever no real id claims the string;
• removing the id-holder no longer severs the other model's cid entry (the shared-map design also corrupted removal).

Inherited from Backbone's design; any graph with generated short ids is exposed.

Tests

• New: get: a model id colliding with another model's cid — pins winning lookup, both models reachable, removal of the id-holder restores the cid string lookup. Fails on master (the colliding model is merged away as a duplicate).
• Updated: the legacy _addReference binds all collection events & adds to the lookup hashes test asserts against _byCid for the cid entry (it inspects the internal maps directly).
• Full client suite: 2113/2113.

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