refactor: split contract_interface.rs into logical modules

[sanity]

Summary

Addresses file bloat in freenet-stdlib by splitting the 1992-line contract_interface.rs into 12 focused modules with clear separation of concerns.

Motivation

This file has been a pain point for navigation and maintenance. While it interfaces with WASM code (making API changes non-trivial), we can safely reorganize the internal structure without breaking the public API.

What Changed

Split rust/src/contract_interface.rs (1992 lines) into a module directory:

Core Type Modules:

• error.rs - ContractError type (19 lines)
• state.rs - State, StateDelta, StateSummary (225 lines)
• key.rs - ContractInstanceId, ContractKey, helper functions (249 lines)
• code.rs - ContractCode (138 lines)
• update.rs - UpdateModification, RelatedContracts, UpdateData, etc. (330 lines)
• contract.rs - Contract type (89 lines)
• wrapped.rs - WrappedState, WrappedContract (183 lines)
• trait_def.rs - ContractInterface trait (88 lines)

Special Modules:

• wasm_interface.rs - WASM FFI boundary, pub(crate) (199 lines)
• encoding.rs - Typed contract helpers, pub (384 lines)
• tests.rs - Unit tests (51 lines)
• mod.rs - Re-exports preserving public API (33 lines)

API Compatibility

✅ No breaking changes - All public types re-exported from mod.rs
✅ Same imports work - use freenet_stdlib::prelude::*; unchanged
✅ All feature flags preserved - #[cfg(feature = "testing")], etc.
✅ All tests pass - 6/6 tests green, builds with --all-features

Benefits

• Easier navigation - Find types quickly by module name
• Clear dependencies - Module structure shows type relationships
• Better maintainability - Smaller files, focused responsibilities
• No churn for users - Public API completely unchanged

Test Plan

cd rust
cargo test --lib contract_interface  # 3/3 tests pass
cargo test --lib                      # 6/6 tests pass
cargo build --all-features            # ✓

Review Notes

This is a mechanical refactoring - code was moved but not modified. The only changes are:

• Adjusting imports (use super::*; instead of use crate::contract_interface::*;)
• Module re-exports in mod.rs
• Removing duplicate feature flags from re-exported types

Requesting @iduartgomez review - This touches the WASM interface boundary, and while the API is preserved, I want sign-off that the module structure makes sense.

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