Replace linear nonce replay scan with normalized hash set

[notforhumansfun-rgb]

Closes #7.

What changed

• deserialize Context.seen_nonces into a deduplicated HashSet<[u8; 32]>
• normalize CLI nonce inputs once into the same binary set
• reject non-0x-prefixed or non-32-byte nonce inputs before verification
• replace the per-verification linear string scan with one O(1) set lookup
• cap CLI input at 100,000 unique consumed nonces and document the boundary
• exercise NONCE_REPLAY with a 100,001-entry set, plus malformed-input and normalization regressions

The JSON wire shape remains string[]; reason codes and normative check order do not change. The public Rust field type changes from Vec<String> to HashSet<[u8; 32]>, matching the issue proposal.

Verification

• cargo build --workspace
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• python crosscheck/verify.py vectors — 9/9
• python -m pytest crosscheck/test_crosscheck.py — 2/2