Separate core and Bitcoin concerns, move BTC state to dynamic field

[Bridgerz]

Summary

Builds on #337. Restructures the Hashi package to separate core MPC/auth infrastructure from Bitcoin-specific logic, then moves BTC state out of the Hashi struct into a dynamic field.

Config split

• Monolithic config.move split into generic config (version gating, pause, upgrade cap) and btc_config (deposit fees, withdrawal params, dust limits)
• BTC defaults injected at init rather than hardcoded in core

Directory reorganization

• Move sources split into core/ (committee, config, treasury, proposals, reconfig, validator, tob, threshold) and btc/ (deposit, withdraw, utxo, btc_config, queues)

BitcoinState dynamic field

• New bitcoin_state module with BitcoinState / BitcoinStateKey structs holding deposit_queue, withdrawal_queue, utxo_pool
• Hashi struct slimmed to chain-agnostic fields; BitcoinState attached via df::add in init
• borrow_bitcoin_state() / borrow_bitcoin_state_mut() accessors on Hashi
• All BTC entry functions access state through the dynamic field
• commit_withdrawal_tx sequences borrows: bridge ops, then treasury burn + config read, then re-borrow for insert

Hashi (shared object)
├── committee_set   ← MPC committee management
├── config          ← version gating, pause, upgrade cap
├── treasury        ← generic coin mint/burn (shared across chains)
├── proposals       ← governance
├── tob             ← threshold-of-broadcast certificates
└── [dynamic field: BitcoinStateKey]
    └── BitcoinState ← deposit_queue, withdrawal_queue, utxo_pool

Rust: move_types::Hashi slimmed to match; scrape_hashi fetches BitcoinState via list_dynamic_fields on the Hashi object.

Test plan

• sui move build — clean
• sui move test — 63 tests pass
• cargo check — compiles
• cargo test -p hashi --lib — 154 tests pass
• cargo clippy / cargo fmt --check — clean
• Move formatting (prettier) — clean