Reduce memory allocations in message parsing hot paths

[gurre]

Changes

Allocation optimizations:

• sync.Pool for Message objects (AcquireMessage/ReleaseMessage)
• Pre-allocate RepeatingGroup.groups slice using expectedGroupSize
• Cache tag ordering closure in RepeatingGroup (was N allocations per Read)

Hot path improvements:

• GetString: removed FIXString intermediate allocation
• ConditionallyRequiredFieldMissing: use strconv.Itoa instead of fmt.Sprintf
• GetTime: fixed recursive RLock by using getBytesNoLock
• Removed unused getFieldNoLock function

Benchmark Results

Benchmark	Before	After	Improvement
GetString	20.7 ns/op, 24 B/op, 2 allocs	9.4 ns/op, 8 B/op, 1 alloc	-54% time, -67% mem
ParseMessageNew	1670 ns/op, 5336 B/op, 28 allocs	—	—
ParseMessagePool	—	482 ns/op, 176 B/op, 2 allocs	3.5x faster, 30x less mem
ParseMessage (reused)	318 ns/op	321 ns/op	no regression
RepeatingGroupRead	—	1198 ns/op, 870 B/op, 11 allocs	—