feat: add prepared statement support (prepare/query_prepared)

[Taure]

Summary

• Add pgo:prepare/2,3 — parse a named prepared statement, returns {ok, Name, ParameterOIDs}
• Add pgo:query_prepared/3,4 — execute a prepared statement by name, skipping PARSE entirely
• Add pgo_prepared_cache module — ETS-based cache for statement metadata
• 13 new Common Test cases

Motivation

pgo currently uses unnamed statements ("") for every query, forcing PostgreSQL to parse and plan each one. Named prepared statements allow skipping the PARSE step on repeated queries, sending only BIND→DESCRIBE→EXECUTE→SYNC.

Benchmarks with erlperf show:

• select_one_by_id: +65% throughput (6,500 → 10,700 ops/sec)
• count(*): +61% (7,300 → 11,800 ops/sec)
• select 100 rows: +20% (1,500 → 1,800 ops/sec)

Usage

%% Prepare once, execute many times on same connection
pgo:with_conn(default, fun() ->
    {ok, _Name, OIDs} = pgo:prepare("my_query",
        "SELECT * FROM users WHERE id = $1"),
    #{rows := Rows} = pgo:query_prepared("my_query", [42], OIDs)
end).

Test plan

• 13 new prepared statement CT cases pass
• All 57 existing tests still pass (70 total)