Add placement occupancy action contracts

[ogyrec-o]

Summary

Adds SDK contracts needed for freven-engine#81 placement occupancy validation.

New API surface:

• BlockWorldView::visit_collision_boxes(...)
  • allocation-free callback access to authored block collision boxes
  • placement validation must use authored collision boxes, not hardcoded Vanilla ids
• PlayerCollisionAabb
• CharacterPhysicsQuery::player_collision_aabb(...)
  • authoritative player body query anchored to the same action input sequence as player_position
• ActionRejectReason::PlacementOccupied
• ActionOutcome::RejectedWithReason(...)
• ClientActionRejectReason::PlacementOccupied

This enables server-authoritative placement/body overlap rejection without weakening authority or putting hardcoded player/block shapes into gameplay handlers.

Non-goals

• Does not implement engine/server validation yet.
• Does not change runtime-loaded guest wire ActionOutcome.
• Does not add client-side prediction admission yet.

Validation

• cargo +stable fmt --all -- --check
• cargo +stable test --locked --workspace
• cargo +stable clippy --locked --workspace --all-targets -- -D warnings
• git --no-pager diff --check