MDBValue Optimizations

[CoreyKaylor]

Optimizations suggested by @sebastienros in https://github.com/CoreyKaylor/Lightning.NET/issues/191
I've only compared to v0.20.0, not the previous comparer implementations. The current branch outperforms the baseline v0.20.0 between 5% and 20%

Separately, it feels like there might be a more optimal route for sorted Guid as well (which I assume is a common key scenario). I'll do a little bit of digging, but if anyone knows something I don't already feel free to share.

Memory

• Allocation-free comparers confirmed - constant 56B overhead regardless of operation count (100 ops → 56B, 10000 ops → 56-59B)

Read Performance - All Comparers (1000 ops, 64B values)

Comparer	Time	vs Native
LengthOnly	24.9 μs	-75%
ReverseSignedInt	99.6 μs	-1%
Default (Native)	100.8 μs	baseline
SignedInt	103.0 μs	+2%
ReverseUnsignedInt	105.8 μs	+5%
UnsignedInt	107.3 μs	+6%
ReverseBitwise	110.8 μs	+10%
Bitwise	113.2 μs	+12%
Utf8String	114.1 μs	+13%
ReverseLength	118.4 μs	+17%
Length	119.4 μs	+18%
ReverseUtf8String	163.9 μs	+63%
HashCode	172.1 μs	+71%

Write Performance - All Comparers (1000 ops, 64B values)

Comparer	Time	vs Native
LengthOnly	82.3 μs	-65%
UnsignedInt	200.4 μs	-15%
SignedInt	202.5 μs	-14%
ReverseLength	219.0 μs	-7%
ReverseSignedInt	221.7 μs	-6%
ReverseUnsignedInt	224.8 μs	-5%
ReverseBitwise	228.6 μs	-3%
Default (Native)	236.5 μs	baseline
Length	237.6 μs	+0.5%
Utf8String	237.8 μs	+0.5%
Bitwise	294.5 μs	+25%
ReverseUtf8String	301.4 μs	+27%
HashCode	314.7 μs	+33%

Integer Keys (10000 ops, 4-byte keys)

Comparer	Time	vs Native
SignedInt	307 μs	-81%
UnsignedInt	312 μs	-81%
ReverseSignedInt	1,012 μs	-38%
ReverseUnsignedInt	1,007 μs	-38%
Default (Native)	1,633 μs	baseline

Notes

• LengthOnly is fastest but only compares by length (no content comparison)
• SignedInt/UnsignedInt provide major gains for integer keys
• HashCode is consistently slowest across all scenarios
• Most custom comparers perform within ±15% of native for general byte data