feat(gateway): root stunned players and show pitch and speed to observers

[Mathih13]

#696 tells the owning client when a root, stun or polymorph holds it, and relays its receipt to peers. Several movement states still did not reach the client. A dead body could walk before release. A ghost could not walk on water. Observers never saw a creature rooted, another player's pitch while swimming, or another unit's sprint or slow speed. A movement acknowledgement on a boat ended the World Session, because the protocol library reads a later client's transport layout. A Game Master or cheat movement request with an unknown opcode or map also ended the session.

This change builds on #696 and keeps one path for each job:

• One root rule. lyracore_shared::aura::holds_in_place decides for the owning client, a creature's observers and the Module's movement gate. The owning client's root now writes the build-5875 packed guid. The library's SMSG_FORCE_MOVE_ROOT writes a full guid, which the client misreads. A dead body stays rooted until release, also when its last hold ends at death. A ghost walks on water (SMSG_MOVE_WATER_WALK / _LAND_WALK). World entry restores both states. Observers see a creature's hold as SMSG_SPLINE_MOVE_ROOT / _UNROOT.
• One relay to peers. fix(movement): roots hold a Character and moving ends Shadowmeld #696 relayed the root receipt as MSG_MOVE_STOP. It now relays as MSG_MOVE_ROOT, with the steering flags still dropped, so the Module's gate accepts it. The unroot receipt relays as MSG_MOVE_UNROOT and the water-walk receipt as MSG_MOVE_WATER_WALK, as cmangos does. A receipt on a transport now relays too. These receipts are not relayed between a same-map teleport and the client's MSG_MOVE_TELEPORT_ACK, because a release acknowledges the unroot at the corpse (cmangos IsBeingTeleportedNear).
• Pitch messages relay to observers like other MSG_MOVE_* movement.
• A slow lowers backward running (SMSG_FORCE_RUN_BACK_SPEED_CHANGE), at world entry and in a peer's CREATE too. Observers get another unit's run, backward run and swim speeds as SMSG_SPLINE_SET_*_SPEED. For players this deviates from cmangos, which relays MSG_MOVE_SET_*_SPEED after the owner's acknowledgement. The motion row holds only a MovementInfo body, so it cannot carry that speed.
• Movement acknowledgements use the build-5875 layout. Cheat, Game Master and minigame movement requests and CMSG_MOVE_FALL_RESET are ignored without decoding, so they never move the Character. The inventory counts them as explicitly ignored.

The importer gives confuse, pacify and charm the polymorph mechanic, so holds_in_place roots them too. Telling them apart needs a distinct mechanic in the imported spell data. The Module change is small: MSG_MOVE_SET_PITCH coalesces like a heartbeat in the motion queue.

New durable tests, left to CI because SpacetimeDB is not run locally:

• benilla_durable_a_stun_roots_the_client_across_reconnect_until_it_ends (Gateway): a stun roots the client through the aura relay, world entry roots it again, and the aura delete unroots it.
• module/tests/pitch_motion.rs: a pitch change coalesces behind a staged transition.

#696's module/tests/movement_interrupts.rs is unchanged. Its Gateway receipt test now checks MSG_MOVE_ROOT. The root receipt constraints #704 moved to docs/benilla-compatibility.md#root-receipts now describe the MSG_MOVE_ROOT relay, the packed-guid root sender and transport receipts.

Verification:

• Local checks: cargo fmt --all -- --check, clippy with -D warnings for the Gateway, Module and shared crates, cargo test -p lyracore-gateway, cargo test -p lyracore-shared, cargo test -p lyracore-module --lib --features=debug_reducers, the wasm check, the inventory example and its tests, and the durable targets with --no-run. All pass.
• Comment review: the whole diff against main (source, tests, the inventory example and the linked doc). Passed. Retained comments are one-line opcode and packet-layout facts, library quirks, the teleport ordering constraint and pointers to docs/benilla-compatibility.md#root-receipts and #movement-controls, which resolve. The removed root_packet contract (no early unroot under overlapping holds) lives in that doc. The other removed comments moved with their code into codec::acknowledged_move.

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