Function joinPath

Append a child path suffix to a parent path. When the suffix is omitted (e.g. transparent wrappers like union options), the parent path is returned unchanged so the child inherits the parent's id.

Bracketed array indices like [0] append directly so tags + [0] becomes tags[0] rather than tags.[0] — matching the canonical form used by core/fieldPath.ts resolvePath, which already parses bracket notation when navigating WalkedField trees.

This is the single authoritative implementation. The copies in react/SchemaComponent.tsx, html/a11y.ts, solid/SchemaComponent.tsx, and vue/idPrefix.ts are kept for backward compatibility but delegate here.