adds an optional tidesdb_pessimistic_locking sysvar (off by default) …

[guycipher]

…that introduces plugin-level striped mutex row locks for high-contention workloads like tpc-c. when disabled, tidesdb uses its native optimistic mvcc semantics with write-write conflict detection at commit time — ideal for low-contention oltp, analytics, and write-heavy workloads. when enabled, row locks are acquired in index_read_map() for exact pk lookups and in update_row()/delete_row() for scan-based mutations, held until commit or rollback. isolation is downgraded to read_committed since row locks already serialize access, avoiding redundant snapshot conflict checks. the stripe array uses xxh3_64bits() from tidesdb's bundled xxhash.h for faster hashing and better distribution on short pk keys, replacing the earlier fnv-1a approach. stripe count is configurable via a new read-only sysvar tidesdb_row_lock_stripes (default 1024, range 64–65536), dynamically allocated at plugin init and freed at deinit. locks are tracked per-transaction in tidesdb_trx_t and released in commit, rollback, and connection close. mtr tests confirm correctness under concurrent updates, two concurrent updates correctly produced d_next_o_id = 3004 and four concurrent updates produced d_next_o_id = 5005; also lastly updated read me