Document terrain interaction contract v2

[ogyrec-o]

Summary

Defines Terrain Interaction Contract v2 as the long-term rc10 contract for voxel terrain break/place interactions.

This is SDK-only design work after freven-engine#386 smoke showed that local prediction fixes alone are not enough: client presented intent and server authoritative validation need one shared contract.

Why

Current Vanilla v1 payloads carry only target position/face/block id, while the client builds intent from prediction-aware presented cursor state and the server reconstructs validation from authoritative world/player position/block-center visibility. That mismatch can produce valid-looking client predictions that later rollback.

What changed

• Adds docs/TERRAIN_INTERACTION_CONTRACT_v2.md.
• Documents intent identity, stream boundary, ray origin/direction, hit/contact semantics, place semantics, prediction dependencies, server validation, deterministic rejection categories, and reconciliation.
• Marks target-position-only v1 semantics as legacy/non-rc10 for the rc10 path.
• Adds SDK vocabulary stubs in freven_block_api without choosing/wiring a serialized payload schema.
• Updates SDK docs/index links and migration notes.

Boundaries

• no engine runtime changes
• no Vanilla runtime changes
• no Boot changes
• no protocol/wire implementation
• no hidden fallback to broken v1 target-pos-only validation
• no client prediction as server truth

Validation

• cargo +stable fmt --all -- --check
• 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
• frevenengine/freven-engine#390
• Use prediction-aware Vanilla block interaction intent freven-vanilla#74
• frevenengine/freven-engine#348