Selective lazy evaluation with targetLength-adaptive gating

[BiplabRaut]

Introduce ZSTD_SELECTIVE_LAZY compile-time option to reduce lazy evaluation overhead in the compression hot path for lazy and lazy2 strategies:

• TargetLength-adaptive match length threshold: skip depth-1/depth-2 re-search when the initial match is strong (>= 12 bytes for low targetLength configurations, >= 16 bytes otherwise), or when the match is a repcode or has a short offset (<= 256)
• Repcode early exit at depth-1 and depth-2: bypass expensive searchMax when a repcode match is already accepted, since regular matches cannot practically beat zero-cost offset encoding

All changes are gated behind ZSTD_SELECTIVE_LAZY. Default build produces upstream-identical output and passes existing deterministic tests.

Detailed Performance Results :

Enable with: make ZSTD_SELECTIVE_LAZY=1