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 html/a11y.ts joinPath and core/fieldPath.ts resolvePath, which already parses bracket notation when navigating WalkedField trees.