fix(ergo-nipopow): reject m == 0 instead of looping forever in best_arg

[Kshot3000]

Summary

Fixes #950. a NipopowProof with m == 0 hangs proof comparison forever. m comes straight from the wire in scorex_parse; nothing checked it, and best_arg's level loop (args.len() >= m) is unconditionally true for m == 0.

The reference implementation guards this in three places, and this PR mirrors all three:

• bestArg has require(m >= 1) → NipopowProof::new and NipopowAlgos::prove now return the new NipopowProofError::ZeroMParameter for m == 0 (mirroring the existing ZeroKParameter for k). The prove path additionally removes a latent panic: with m == 0 it indexed sub_chain[sub_chain.len() - m] on a non-empty sub-chain.
• NipopowProof.isValid checks PoPowParams.isValid(m, k) (m >= 1 && k >= 1 && m + k <= Int.MaxValue) first → is_valid() now does the same via a new has_valid_params(). Parsed proofs with invalid parameters are simply invalid, so is_better_than never reaches best_arg with them (it returns self.is_valid(), matching the reference's fallback).
• As defense in depth for direct best_arg callers, the level loop also stops when no header reaches the current level: with an empty args, every higher level is empty too. This changes nothing for m >= 1 (there args.len() >= m already implies non-empty).

Verification

Fail-first: on pristine develop a minimal m = 0 proof (single suffix-head header, empty interlinks/proof, so the other validity checks pass) never returns from is_better_than — killed after a 25 s timeout.

New regression tests (ergo-nipopow, --all-features):

• zero_m_proof_is_invalid_and_never_better — the hanging call now returns Ok(false); the proof is invalid solely because of its parameters.
• verifier_processes_zero_m_proofs — NipopowVerifier::process returns for a second m = 0 proof instead of hanging in the comparison.
• new_rejects_zero_m_and_zero_k — NipopowProof::new returns ZeroMParameter / ZeroKParameter.
• prove_rejects_zero_m — prove returns ZeroMParameter instead of panicking.
• best_arg_zero_m_empty_chain — best_arg(&[], 0) returns 0 (the diverging-chain shape the comparison actually produces).

Suites: ergo-nipopow --all-features 6/6 (incl. the existing roundtrip proptest), ergo-lib --all-features green; clippy -D warnings clean; fmt clean.

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Submitted by @Kshot9000 (https://x.com/kshot9000) — independent Ergo builder.
Tips: ERG 9fcM5RWnAjmP4vx5bnW6yohB6H9bLq8sJbaPLHtwZLtQPB32Pvy