[neeerp]

Row 205 S5b (report b177 R5-R6): the rest of a hidden GM's absence.

Mobs (R5):

• FieldActor::mob_candidate picks a controller who isn't hidden. Spawns, revivals, carnival summons, the dojo and Horntail use it, and so does a leaver's hand-off. A hidden joiner gets no mobs.
• A hidden GM's hits (provoke) and distance reports don't take control. Neither do the mobs the GM brings out with a summoning sack or a script (mover_for).
• Hiding hands the GM's mobs to someone else, telling the GM's client to stop moving them (MobSet::hand_off, which keeps the GM's damage credit). Unhiding gives the GM the mobs no one moves.
• TakeDamageHandler ignores every hit to a hidden GM, as Cosmic does.

Directory (R6):

• Presence carries gm and hidden, kept current by sync_presence.
• whisper, find, rock_locate, morph_another and family_locate answer "not found" to an asker who isn't a GM.
• Buddy lists, guilds and parties still show the GM (R7, unchanged).

Docs: agent_docs/server-architecture.md (!hide).

Tests:

• New unit tests: mobs_pass_by_a_hidden_gm, only_gms_find_or_whisper_a_hidden_gm, and a hidden-GM case in snails_hurt_up_to_their_bound.
• runtime and net lib tests with skill data: 1231 + 227 passed.
• Harness: gm_skills, mob_combat, mob_skills, find_player, whisper_between_two_players, teleport_rocks, party_potions and damage_reductions all pass.
• One Sonnet review. Its finding (sack and script mobs went to a hidden GM) is fixed in this PR.