feat: PostgreSQL type OID mapping + encode/decode

[FumingPower3925]

Summary

Implement PostgreSQL type OID mapping and value encoding/decoding for the most common data types. This bridges Go types to PostgreSQL wire format.

Design

Package: driver/postgres/protocol/types.go

Core Type OIDs

OID	PostgreSQL Type	Go Type
16	bool	bool
20	int8	int64
21	int2	int16
23	int4	int32
25	text	string
700	float4	float32
701	float8	float64
1043	varchar	string
1082	date	time.Time
1114	timestamp	time.Time
1184	timestamptz	time.Time
17	bytea	[]byte
2950	uuid	[16]byte / string
3802	jsonb	[]byte / json.RawMessage
1700	numeric	string (or big.Rat)
1009	text[]	[]string
1016	int8[]	[]int64

Encoding (Go → PG wire)

type Encoder struct{}

func (e *Encoder) Encode(oid uint32, value any, buf []byte) ([]byte, error)
func (e *Encoder) EncodeBinary(oid uint32, value any, buf []byte) ([]byte, error)
func (e *Encoder) EncodeText(oid uint32, value any, buf []byte) ([]byte, error)

Decoding (PG wire → Go)

type Decoder struct{}

func (d *Decoder) Decode(oid uint32, format int16, data []byte) (any, error)
func (d *Decoder) DecodeBinary(oid uint32, data []byte) (any, error)
func (d *Decoder) DecodeText(oid uint32, data []byte) (any, error)

Binary Format Details

• int2/int4/int8: big-endian fixed-width
• float4/float8: IEEE 754 big-endian
• bool: single byte (0/1)
• text/varchar: raw UTF-8 bytes
• timestamp: microseconds since 2000-01-01 00:00:00 UTC
• date: days since 2000-01-01
• bytea: raw bytes
• uuid: 16 raw bytes
• arrays: array header (ndim, flags, element OID) + dimensions + elements

database/sql.Scanner / driver.Valuer Support

• Types implementing driver.Valuer are encoded via their Value() method
• Scan targets implementing sql.Scanner receive decoded Go values

Acceptance Criteria

• OID constants for all common types
• Text encoding/decoding for all listed types
• Binary encoding/decoding for int, float, bool, text, bytea, timestamp, uuid
• Array encoding/decoding (text[] and int8[] at minimum)
• NULL handling (nil → SQL NULL, SQL NULL → nil)
• driver.Valuer and sql.Scanner interface support
• Unit tests with known PG binary representations
• Benchmark: encode/decode throughput for common types

Dependencies

• Depends on 123 (wire protocol framing — uses Writer/Reader)