Split expensive tests based on n={1, 2, 3}

[mariaKt]

This PR adds test functions with assumptions in order to split expensive multisig proofs into per-N variants (n=1, n=2, n=3)

The most expensive multisig proof tests (burn, burn_checked, transfer, transfer_checked, set_authority_account, set_authority_mint) each explore all three values of N (number of registered signers) in a single proof. The validate_owner signer-checking loop produces an N=1/N=2/N=3 cascade, multiplying the combinatorial path count and making these proofs very large (40+ hours, high memory usage).

This PR adds _n1, _n2, _n3 variants of each test function that constrain multisig.n to a specific value instead of the range 1..MAX_SIGNERS. Each variant replaces:

if multisig.n < 1 || multisig.n > MAX_SIGNERS_U8 {
    return Ok(());
}

with:

if multisig.n != 1 {  // or 2, or 3
    return Ok(());
}

The rest of the function body is identical. The three per-N proofs together cover the same state space as the original combined proof.

Functions added (18 total, 3 per test):

• test_process_burn_multisig_n1/n2/n3
• test_process_burn_checked_multisig_n1/n2/n3
• test_process_transfer_multisig_n1/n2/n3
• test_process_transfer_checked_multisig_n1/n2/n3
• test_process_set_authority_account_multisig_n1/n2/n3
• test_process_set_authority_mint_multisig_n1/n2/n3

Each variant is added to the same spec file as the original function and referenced in use_tests in both p-token and spl-token entrypoints to ensure inclusion in the smir json.

Expected benefits:

• The n=3 path is definitely the most expensive, but each per-N proof is definitely more manageable compared to the combined proof (fewer splits, less memory)
• Per-N proofs can run in parallel on the online server
• Individual proofs are less likely to OOM
• The M threshold remains symbolic (1..MAX_SIGNERS) in all variants, but we could choose to further reduce branching by restricting this in favor of more variants as well.

Original functions are preserved — the combined test_process_*_multisig functions remain unchanged, for dispatch.

Also included: A minor fix adding a missing doc comment on MAX_SIGNERS in interface/src/instruction.rs (pre-existing lint error for spl-token builds).