Add optional proof-of-work grinding plumbing

[latifkasuli]

Depends on #225. This branch includes its exact commit so the grinding code can be tested now; that dependency diff drops after #225 lands.

What changed

• Adds a BLAKE3 proof-of-work nonce primitive to the transcript.
• Adds ZipTypes::GRINDING_BITS, defaulting to 0.
• Writes/checks the nonce after the bound combined_row and before column indices are derived.
• Covers roundtrip, tampering, zero-bit behavior, invalid settings, proof-size accounting, and default/parallel execution.

Scope

This is parameter-neutral plumbing. It does not reduce the production column-opening count, change the benchmark configuration, or claim that g grinding bits can automatically replace g queries.

Any query reduction needs a protocol soundness argument that isolates the query error term from the other failure terms. Until then, the default remains off and the non-test configurations remain unchanged.

Validation

• cargo fmt --all -- --check
• cargo test -p zinc-transcript --lib (9 passed)
• cargo test -p zinc-transcript --lib --features parallel (9 passed)
• cargo test -p zip-plus --lib (93 passed)
• cargo test -p zinc-protocol --lib (30 passed)
• cargo test -p zinc-protocol --lib --features parallel (30 passed)

Related to #149.