Add client presented terrain read API

[ogyrec-o]

Summary

Adds a client-presented terrain read to ClientCameraHitProvider.

Why

frevenengine/freven-engine#386 exposed that gameplay code needs to validate and retarget client-side interactions against the same presented terrain stream that the engine uses for prediction/reconciliation.

The previous API exposed only:

• authoritative terrain reads
• predicted terrain reads

That forced Vanilla to build a separate thread_local pending terrain bridge to cover dependent clicks like LMB -> RMB. That is the wrong ownership boundary: the engine owns action_seq, predicted edits, ActionResult reconciliation, and WorldDelta close.

What changed

• Adds ClientCameraHitProvider::presented_block_id_at(owner, pos).
• Provides a source-compatible default implementation that delegates to predicted_block_id_at.
• Documents the intended semantics: implementations may compose authoritative terrain with owner-local predicted edits still pending reconciliation.

Boundaries

• no engine source changes
• no Vanilla source changes
• no Boot changes
• no protocol changes
• no server authority weakening
• no client prediction as authority

Validation

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

Related:

• frevenengine/freven-engine#386
• frevenengine/freven-engine#395
• Keep local terrain predictions through grace window freven-vanilla#88
• Retain local pending terrain predictions briefly freven-vanilla#87
• Add ordered mouse press drain API #118