feat: typed queries and results

[hpopp]

Added

• execute caches prepared statements per connection, keyed by SQL text and parameter types. The cache holds Config.statement_cache_size statements (default 256) with least-recently-used eviction. Set 0 to turn the cache off. Repeated statements skip the server parse and plan step.
• A cached statement that a schema change invalidated (SQLSTATE 0A000), or that the server dropped (SQLSTATE 26000), is prepared again and retried once instead of surfacing the error.
• Value gains opt_bool, opt_int, opt_float, and opt_string constructors that map Option values to SQL NULL, plus as_* and *_or accessors for reading result columns.
• The params argument of Connection.execute defaults to no parameters, so statements without placeholders need no empty list.

Changed

• Breaking change Result rows are now typed. QueryResult.rows is List<List<Postgres.Value>> instead of List<List<Option<String>>>. Rows decode from the column type OIDs the server reports. Types without a first-class variant (numeric, timestamps, uuid, json) pass through as Value.String, and SQL NULL decodes to Value.Null.
• Breaking change Connection.execute takes List<Postgres.Value> parameters instead of List<Option<String>>. The driver declares parameter types in the Parse message (Value.Int as int8, Value.Bool as bool, Value.Float as float8), so most $1::type casts in SQL are no longer needed. Bind SQL NULL with Value.Null.