+ implement greedy and exhaustive byte-pair compression

[Hawkynt]

What changed

• replace the existing 16-bit BPE demonstrator with Philip Gage-style byte-pair compression
• keep encoded symbols byte-sized by substituting pairs with byte values absent from each block
• keep the parameterless/default building block on deterministic greedy construction with 65,535-byte blocks
• add BpeConstructionStrategy.Exhaustive, which memoizes every profitable merge sequence within 64-byte exact-search blocks and chooses the smallest serialized grammar
• keep greedy and exhaustive output on the same wire format; either instance can decode the other's stream
• fall back to raw storage whenever a constructed block is not smaller
• reject malformed block framing, duplicate codes, forward/cyclic grammar references, length mismatches, and trailing data
• classify BPE as a dictionary/grammar compressor rather than an entropy coder

Verification

Added NUnit coverage for:

• empty input
• repetitive single-byte data and ratio
• repeated multi-byte phrase and ratio
• random incompressible input
• all 256 byte values / raw fallback
• multi-block boundary round-trip
• deterministic greedy wire vector
• a non-greedy 13-byte counterexample where exhaustive construction beats greedy/raw fallback
• cross-decoding between greedy and exhaustive streams
• exhaustive search-block boundary round-trip
• malformed forward/cyclic grammar rejection
• stable registry metadata and default strategy

The previous head completed the Ubuntu and Windows build/core/interop/OS/polyglot/performance jobs successfully. The local execution environment does not have the .NET SDK installed, so the new exhaustive-strategy commit is compiled and tested by repository CI.