feat(contracts): Wasm bytecode size optimization via sub-contract mod…

[devJaja]

feat(contracts): Wasm bytecode size optimization via sub-contract modular architecture

Problem

Soroban enforces a strict Wasm bytecode size limit per contract. The btc_relay contract exceeded this limit due to
its inline Bitcoin cryptographic helpers (double-SHA256, Merkle proof traversal, PoW target decoding).
Additionally, 7 of the 11 contracts were missing from the workspace, making them unbuildable.

Solution

Extracted the heavy crypto logic from btc_relay into a new btc_relay_crypto sub-contract. The relay contract now
delegates all crypto operations via #[contractclient] cross-contract calls, keeping each Wasm binary well within
Soroban's size limit.

Changes

• contracts/Cargo.toml — added all 7 missing contracts to the workspace (btc_relay, btc_relay_crypto,
  flash_loan_guard, htlc, lending_liquidation, multi_hop_swap, rbac)
• contracts/btc_relay_crypto/ — new sub-contract housing double_sha256, extract_merkle_root, extract_target,
  hash_meets_target, compute_merkle_root
• contracts/btc_relay/src/lib.rs — removed inline crypto helpers; added BtcCryptoClient cross-contract interface;
  Config now includes crypto_contract: Address
• contracts/btc_relay/src/test.rs — updated tests to register the crypto sub-contract and pass its address to
  initialize
• contracts/btc_relay/Cargo.toml — added btc_relay_crypto as a dev-dependency
• contracts/relayer_slashing/src/lib.rs — fixed pre-existing moved-value compile error
• contracts/por_validator/src/lib.rs — fixed pre-existing invalid token::Client::total_supply() call; oracle now
  owns circulating_supply

Deployment note

btc_relay_crypto must be deployed first. Its contract address is then passed as crypto_contract when initializing
btc_relay.

Testing

cargo check --workspace passes clean across all 13 contracts.

Closes: #270

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