feat: is token mint classifier

[sonicfromnewyoke]

Problem

classifying accs as Mints on the hot path of an indexer or unfiltered getProgramAccounts pipeline is expensive if done via a full borsh decode - hundreds of nanoseconds and dozens of allocations per candidate, most of which are discarded.
#396 proposes a byte-level IsTokenMint(acc *Account) bool helper, but the submitted version has a latent nil-panic on acc == nil, uses magic numbers, and orders its checks so every call pays for a 32-byte PublicKey comparison - including the majority case where the account isn't a token at all

Summary of Changes

• added helper with the same API and logic as proposed in Add IsTokenMint helper #396
• added test-cases
• performed benchmarks, to not propose worse solution - 1.86x faster on mixed indexer workloads, 1.77x on cold path, 0 allocations

Benches

Scenario	switch-on-owner	length-first	Speedup
Mixed 8-shape rotation (indexer)	3.51 ns	1.89 ns	1.86x
Hot path - bare SPL Token mint	2.68 ns	2.47 ns	1.09x
Cold path - non-token owner	3.15 ns	1.78 ns	1.77x

supresses #396