fix!: Execute queries inside a Transaction

[ValuedMammal]

Fixes #17

Description

Every write path previously executed N individual SQL statements directly against the connection pool, each running in its own implicit auto-commit transaction. If any statement failed mid-way, the database was left in a partially-applied state, e.g. a ChangeSet whose tx_graph was written but whose local_chain was not.

This PR wraps all writes in an explicit BEGIN / COMMIT transaction so that a ChangeSet either lands completely or not at all.

Notes to the reviewers

wallet: Store::write_changeset now opens a single sqlx transaction and threads it through every write method (write_network, write_keychain_descriptors, write_local_chain, write_tx_graph, write_keychain_txout, write_locked_outpoints). The transaction is committed only after all writes succeed; any error causes an implicit rollback.

async_store: The lower-level write methods (write_tx_graph, write_local_chain, write_keychain_txout) and their read counterparts now accept conn: &mut SqliteConnection rather than operating on &self.pool. This makes them usable inside a caller-owned transaction without leaking transaction management concerns into the individual methods.

The performance improvement is a secondary benefit. SQLite still amortises WAL-frame overhead across the single COMMIT, reducing internal bookkeeping compared to N auto-commit boundaries.

Changelog

BREAKING — all lower-level read/write methods that previously took &self are now member functions accepting conn: &mut SqliteConnection. Callers who invoked these directly must now supply a connection or transaction handle.

async_store:

• Store::write_tx_graph
• Store::write_local_chain
• Store::write_keychain_txout
• Store::read_tx_graph
• Store::read_local_chain
• Store::read_keychain_txout

wallet:

• Store::write_network
• Store::write_keychain_descriptors
• Store::write_locked_outpoints
• Store::read_network
• Store::read_keychain_descriptors
• Store::read_locked_outpoints

The high-level entry points Store::write_changeset and Store::read_changeset are unchanged and remain the recommended API.