Fix _idSeq global mutation on diagram load

[zntznt]

Problem

_idSeq (js/model.js:307) is a module-level mutable global. Diagram.loadJSON() at line 826 advances it:

_idSeq = Math.max(_idSeq, data._idSeq || 0);

Two Diagram instances in the same process share this counter. Loading a diagram with a high _idSeq permanently skews ID generation for all future diagrams — including Monte Carlo clones and undo snapshots.

Fix

Remove the global mutation. Store _idSeq per-Diagram:

// Diagram.loadJSON, line 826 — change:
_idSeq = Math.max(_idSeq, data._idSeq || 0);
// to:
this._idSeq = Math.max(this._idSeq || _idSeq, data._idSeq || 0);

Add _idSeq initialization in the Diagram constructor.

The module-level _idSeq still exists for genId() calls outside a diagram context (tests, demos), but loading a diagram no longer mutates it globally.

Effort

~10 minutes. 1 file (js/model.js). 3 lines changed.

[zntznt]

Implementation Order

This is Phase 3 · Step 3.4 — implement last (trivial, lowest priority fix).

Priority: LOW
Phase: 3 (Structural / Developer-Facing)
Order: 10 of 10
Depends on: Nothing
Effort: ~10 minutes