test(engine): close the t105 ruling riders — soulbond pin, dangling-id pin, save-compat round trip

[matthewevans]

Three pins demanded by the ruling on the t105 checkpoint. Test-only; no production
change.

1. purged_token_soulbond_source_must_not_pair_cr_702_95b — soulbond is the largest
   slice of the class (50 of 91 defs, 25 of 65 faces) and MUST stay false for a source
   that has ceased to exist (CR 702.95b: pairing exists only between creatures on the
   battlefield). It stays false structurally today, but structural immunity is one
   refactor away from gone, so pin the behavior rather than the mechanism.

   Soulbond's triggers are synthesized from the keyword during the card-data build, not
   by parse_oracle_text, so they cannot be pulled off an oracle-synthesized object the
   way Dreampod Druid's are (the inline-keyword harness foot-gun). The filter is therefore
   constructed — and then asserted BYTE-IDENTICAL to the filter all 50 soulbond defs
   actually carry in card-data. That premise guard is what keeps a constructed filter from
   becoming a fabricated one: if the real shape drifts, the premise fails loudly instead of
   the pin passing against a filter no card has. Carries a non-vacuity leg (a live unpaired
   creature MUST match the same filter).

2. lki_attachment_ids_are_identity_only_and_survive_a_purged_aura — the ruling flagged the
   ObjectId inside LKISnapshot::attachments as a possible dangling-reference trap. It is
   not: att.object_id is used in exactly two places in the filter layer, and both are pure
   identity comparisons (== source_id / != source.id, for exclude_source); it is never
   fed to state.objects.get(). The kind and controller the gate reads are stored by
   value. Pinned by destroying the Aura object outright after the host is purged, so the
   stored id dangles — and HasAttachment must still answer TRUE from the snapshot without
   a panic. (AttachmentSnapshot is also already persisted today via
   ZoneChangeRecord::attachments, so this is a reused shape, not a new surface.)

3. lki_snapshot_attachments_are_save_compatible_both_directions — GameState is
   Serialize/Deserialize and owns lki_cache, so LKISnapshot IS persisted. OLD → NEW: a
   pre-change save with no attachments key must still deserialize and default to the empty
   set (the pre-change fail-closed answer, so a legacy save cannot start fabricating
   attachment matches). NEW → NEW: a snapshot carrying attachments must survive a full
   GameState round trip intact.