feat: RESP2/RESP3 parser (zero-alloc)

[FumingPower3925]

Summary

Implement a zero-allocation RESP2/RESP3 protocol parser for the Redis driver. This is the foundation for all Redis communication.

Design

Package: driver/redis/protocol/resp.go

RESP2 Types

Prefix	Type	Example
+	Simple String	+OK\r\n
-	Error	-ERR unknown command\r\n
:	Integer	:1000\r\n
$	Bulk String	$5\r\nhello\r\n
*	Array	*2\r\n$3\r\nfoo\r\n$3\r\nbar\r\n

RESP3 Additional Types

Prefix	Type	Example
_	Null	_\r\n
#	Boolean	#t\r\n
,	Double	,3.14\r\n
(	Big Number	(123456789\r\n
!	Blob Error	!21\r\nSYNTAX invalid\r\n
=	Verbatim String	=15\r\ntxt:Some string\r\n
~	Set	~2\r\n+a\r\n+b\r\n
%	Map	%2\r\n+key1\r\n:1\r\n+key2\r\n:2\r\n
|	Attribute	(metadata prefix)
>	Push	>3\r\n+message\r\n+channel\r\n$5\r\nhello\r\n

Parser

type Reader struct {
    buf  []byte
    pos  int
}

type Value struct {
    Type    byte      // prefix byte
    Str     []byte    // for string/bulk string/error (slice of buf, no alloc)
    Int     int64     // for integer
    Float   float64   // for double
    Bool    bool      // for boolean
    Array   []Value   // for array/set/push
    Map     []KV      // for map
    IsNull  bool
}

type KV struct {
    Key   Value
    Value Value
}

func (r *Reader) Feed(data []byte)
func (r *Reader) Next() (Value, error)  // returns ErrIncomplete if partial
func (r *Reader) Reset()

Zero-Alloc Strategy

• Value.Str is a slice into Reader.buf (no copy)
• Value.Array uses a pre-allocated slice pool
• Integer parsing without strconv.Atoi (inline digit loop)
• No interface{} boxing — typed fields on Value struct

Command Serialization

type Writer struct {
    buf []byte
}

func (w *Writer) WriteCommand(args ...string) []byte
func (w *Writer) WriteBulkString(s string)
func (w *Writer) WriteArray(n int)
func (w *Writer) Reset()

Serializes as RESP2 array of bulk strings (universal command format).

Acceptance Criteria

• RESP2 parser handles all 5 types correctly
• RESP3 parser handles all additional types
• Nested arrays/maps parsed correctly
• NULL bulk string and NULL array handled
• Partial message returns ErrIncomplete (resumable)
• Zero allocations on parse hot path (benchmark verified)
• Command serialization produces valid RESP
• Fuzz tests for parser robustness
• Benchmark: parse throughput (MB/s) and allocs/op

Dependencies

None — standalone protocol implementation.