Cherry-pick rightAngle minPathMargin (#3266) into dev

[kumilingus]

Summary

• Cherry-pick 858d04955 from master: feat(router): add minPathMargin option to rightAngle router to suppress margin-overlap detours (feat(router): add minPathMargin option to rightAngle router to suppress margin-overlap detours #3266)
• The other requested commit (6363f39ac — Cell.toJSON predicate) is already in dev via feat(joint-react): paper-transform, linkRouting, useGraph JSON helpers + Cell.toJSON predicate #3298, so it was skipped (empty cherry-pick).

Notes

• Conflict in test/jointjs/routers.js was resolved by keeping only the minMargin tests introduced by the cherry-picked commit. The trailing context (rightAngle routing with source anchor outside the element bbox test) belongs to commit 2a1fec504 (fix(routers.rightAngle): include anchor in bbox union #3288), which is not part of this cherry-pick and not yet in dev.

Test plan

• yarn test-server — QUnit router tests pass
• yarn test-ts — types compile
• yarn lint