v83 move paths for players and mobs; server-driven NPC idle actions

[neeerp]

Movement now travels as v83 move paths in both directions, mobs move by paths their controller builds, and NPC idle actions go through the server.

What changed

• Path elements are read and written by type as v83 does (FUN_0068a33c decode, FUN_0068a563 encode):
  • velocity-only types keep the position before them, type 10 is one byte, and every other type ends with stance and duration;
  • HeavenClient sends velocities in px/s and a real start point, and records element 4 for Teleport;
  • the server relays the client's start point instead of 0, for players and mobs.
• Attack action: the attack packets' action is a short with the facing in the top bit.
• Batching: the user's steps merge into runs and go out about once a second (FUN_0068ab85, FUN_0068a88d). Air and rope elements wait until a foothold follows. Other characters play whole paths on a Hermite curve, a tenth faster when a second behind and snapping when over six (FUN_0068b371, FUN_0068adcc).
• Mob paths: the controller runs the mob's AI ahead of what is shown, steps it until it has a second of movement (FUN_0066b6fc), sends that path in MOVE_MONSTER and plays it like every other client.
• NPC control: the server gives one player per field control of its NPCs (0x103, handed on when they leave) and relays their idle choices (0xC5) to the map as 0x104. HeavenClient chooses only for NPCs it controls and plays what comes back (FUN_006d2918, 006d2e07). It also handles REMOVE_NPC and control release.

Not covered

• Walking NPC paths: no v83 template has info/move.

Verification

• New client test movement_path.
• New server unit tests cover the path layouts; the harness's strict packet loops now skip NPC control (0x103).

Reviewed with sol-loop: 2 rounds with gpt-6.1-sol on high.

Stacked on #503