+ add FPAQ0 order-0 arithmetic building block

[Hawkynt]

What changed

• add Fpaq0BuildingBlock as a missing entropy-coding primitive
• reuse the existing binary arithmetic encoder/decoder with FPAQ0-style order-0 byte-prefix contexts
• initialize each binary branch uniformly, adapt after each bit, and rescale counts before they grow large
• keep the building-block envelope self-delimiting with a 4-byte original-length field
• document the FPAQ-family reference and distinguish the CompressionWorkbench framing from an FPAQ archive file

Verification

Added NUnit coverage for:

• empty input
• highly repetitive data and compression ratio
• repeated English phrase and compression ratio
• random incompressible data
• all 256 byte values
• deterministic encoded vector
• missing payload and negative-length rejection
• stable registry metadata

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