refactor: unify/cleanup methods of treecrdt in core package

[Antonov548]

Refactor: unify apply_remote and simplify TreeCrdt API

Reduces API surface of treecrdt-core by merging three remote-apply methods into one unified apply_remote, removing dead code, and extracting helpers into dedicated modules.

Key changes

• Unified apply_remote: merged apply_remote(), apply_remote_with_delta(), and apply_remote_with_materialization_seq() into a single apply_remote() -> Result<Option<ApplyDelta>>. Materialization behavior is driven by a new ParentOpIndex type parameter on TreeCrdt (defaults to NoopParentOpIndex — compiles to no-ops for in-memory/WASM usage).
• Simplified finalize_local: removed external &mut index parameter; uses the embedded index.
• Single local-op method per type: consolidated duplicated base/with_plan variants of local_insert, local_move, local_delete, local_payload.
• Extracted helper modules: moved types to types.rs, affected-node helpers to affected.rs, validation to validation.rs — tree.rs reduced from 1200 to 780 lines.
• Removed dead code: unused methods, redundant constructors, excess visibility.
• Replaced PersistedRemoteStores with MaterializationStores struct used by both materialize_persisted_remote_ops_with_delta and catch_up_materialized_state.

Adapter impact

• WASM: no type signature change (defaults apply); both appendOp/appendOpWithDelta call unified apply_remote.
• SQLite/Postgres: session TreeCrdt now constructed with the real ParentOpIndex impl; finalize_local calls simplified.