fix: set parents for all variadic inserted nodes

[feritcemkahraman]

When insertBefore or insertAfter receives multiple new nodes, only the first node gets a parent. Later nodes are serialized but their remove() call does nothing, and sibling navigation cannot resolve the container.

Assign the parent to each additional node. Add regressions for both insertion methods that verify the parent and subsequent removal.

Validation: both new tests fail before the fix; all 798 tests pass after it. Build, TypeScript checks, and lint pass (lint reports two existing warnings in exceptions.mjs).