Deleting a type inside a branch clears main's model and cache #772

Proposed Changes

Problem: Deleting a type inside a branch unregisters main's model and throughs and clears main's cache for every branch.

Fix: Guard the unregister block with if not in_branch; clear only the current context.

Where (line numbers as of a61cd70; they will have drifted): models.py:2235, 2322–2335

Audit: finding E3, severity Medium, effort S, originally batch 5 (#737). Not re-checked after the review: first confirm it against current main (reproduce it, or point to the code) before fixing.

Note: Related but separate: #755 / #756 handle a type deleted in main leaving its model registered in other workers. That fix deliberately doesn't signal for deletes inside a branch, so this in-process problem remains.

Justification

A user-reachable correctness or data-integrity bug, so it blocks v1.0. Tracked in #759 under tier 1 (correctness and data integrity), re-sliced from the pre-1.0 code audit so each fix gets its own reviewable PR.