Add shared terrain interaction v2 validator

[ogyrec-o]

Summary

Adds the shared Terrain Interaction Contract v2 validator to freven_block_api.

This makes the SDK the legal shared owner of terrain break/place validation semantics so engine and Vanilla can use one implementation without dependency inversion or duplicated validator math.

Why

frevenengine/freven-vanilla#75 blocked runtime v2 wiring because the validator lived in freven_client_engine. Vanilla must not depend on engine internals, and copying the validator into Vanilla would create a second authority implementation and risk divergent server validation semantics.

What changed

• Adds freven_block_api::terrain_interaction.
• Exposes:
  • validate_terrain_interaction_v2
  • TerrainInteractionValidationPolicyV2
  • TerrainInteractionValidationV2
  • TerrainInteractionAcceptedV2
  • TerrainInteractionWorldViewV2
  • TerrainInteractionRulesV2
  • TerrainInteractionCellV2
  • BlockWorldViewTerrainAdapter
• Reuses existing SDK v2 vocabulary:
  • TerrainInteractionIntentV2
  • TerrainInteractionKindV2
  • TerrainInteractionRayV2
  • TerrainInteractionHitV2
  • TerrainPlaceIntentV2
  • TerrainInteractionRejectReasonV2
• Adds deterministic tests for ray sanity, reach, first-solid traversal, face/contact validation, place support/cell validation, duplicate action/dependency checks, and client-ray-origin-not-authority.
• Documents freven_block_api::terrain_interaction as the long-term shared validator owner.

Boundaries

• no engine runtime changes
• no Vanilla runtime changes
• no Boot changes
• no protocol/wire payload implementation
• no client prediction as server truth
• no block-center-only validation
• no hidden v1 fallback

Validation

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

Related:

• frevenengine/freven-engine#386
• Document terrain interaction contract v2 #116
• frevenengine/freven-engine#391
• Document terrain interaction v2 runtime wiring blocker freven-vanilla#75
• frevenengine/freven-engine#348