fix(core): reject entries with no parent_id instead of building relative paths

addressing.rs: when resolving SdPath::Physical, entries with parent_id=None
produced a relative path like "file.txt" which violates the SdPath::Physical
contract (must be absolute). Now returns an error instead. In practice this
case shouldn't occur (root entries are directories), but it's correct to
fail explicitly rather than silently produce invalid paths.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>