v1.18.0

Two behaviour improvements to arch suggest (suggestTopology), requested by the downstream
ArchCanvas product, which persists a chosen candidate's insertText back into .arch source. Both
are unconditional — per ADR 0005 suggestions
are deterministic data, and the new behaviour is strictly more correct, so there is no opt-in flag.
The public Suggestion / SuggestionCandidate types are unchanged; only the string a candidate
carries (and, for one fault, candidate order) changes.

Changed

• Candidates reference walls only by a STABLE ref. A candidate's insertText used to be able to
  name a wall by its positional auto-id (e.g. door on partition_3 at 50%), which re-indexes
  silently when a later edit inserts an earlier same-category wall — corrupting any persisted
  suggestion. Each candidate now composes its placement as the first available of: an author-declared
  wall id (on <id>), a unique wall category (on <category>, valid iff exactly one wall carries
  it), or absolute coordinates (at (x, y), which name no wall — the compiler's nearest-wall
  hosting binds the intended one). A positional auto-id is never emitted. Applies to all four
  builders (entrance, unreachable-room, bedroom-window, bath-via-bedroom).
• A private unreachable room prefers reconnecting inward. For W_ROOM_UNREACHABLE on a private
  room (bedroom or wet room), interior candidates that reconnect to a space already reaching the
  entrance now rank above exterior (new-outside-door) candidates, regardless of run length; within
  each group the existing longest-free-run order is kept. Non-private rooms keep the pure geometric
  order. W_NO_ENTRANCE (exterior-first by design), W_BATH_VIA_BEDROOM (already interior-preferred),
  and W_BEDROOM_NO_WINDOW (windows are exterior-only) are unchanged.

Internal

• RWall carries an internal _idAuthored marker (set in resolve from assignIds' knowledge of
  whether the id was author-declared; _-prefixed, never serialized into the Scene/exports), so
  suggestTopology can tell an authored id from an assigned positional one.