Mudlet consistency for transient data

• recompute mAreaExits on every writeMap / writeMapToBuffer call so cross-area routing data stays in sync with the room graph. Mirrors Mudlet's TArea::determineAreaExits: cardinal exits encode as [targetId, DIR_*] with DIR_NORTH=1..DIR_OUT=12, string-named special exits encode as [targetId, DIR_OTHER=13], and dangling or same-area exits are excluded.
• recompute per-area spatial extents (zLevels, min_x/max_x/min_y/max_y/min_z/max_z, and the per-Z xminForZ/xmaxForZ/yminForZ/ymaxForZ maps) on every writeMap / writeMapToBuffer call so Mudlet's renderer gets the right bounding box and Z-level list after any editor mutation. Mirrors TArea::calcSpan in Mudlet's src/TArea.cpp, including two C++ quirks: y is negated when stored on the area (area.min_y = -room.y), and the span / pos QVector3D fields are deliberately left untouched because C++ never writes them either. Areas with zero rooms have their per-Z maps and zLevels cleared but their min/max preserved, again matching C++.
• recompute mpRoomDbHashToRoomId on every writeMap / writeMapToBuffer call so the room content-hash index stays in sync with the room graph. Mirrors TRoomDB::hashToRoomID in Mudlet's C++ source. On load, room.hash is populated from the inverse of the on-disk index (the binary format carries hashes only on the map, not per room), so callers can treat room.hash as the source of truth. On save, the index is rebuilt from each room's hash field: rooms with undefined / '' / null are skipped, and colliding hashes log a warning with last-write-wins. mRoomIdHash (per-profile player cursor) is deliberately left alone — do not confuse the two.
• callers that were maintaining mAreaExits, per-area spatial extents, or mpRoomDbHashToRoomId by hand will have their values overwritten — the computed values are now authoritative.