fix(spammer): reject too many exponential generators instead of overflowing

[Jr-kenny]

Fixes #137

Problem

The guard in partition_exponential that should reject more generators than the account space can hold computes 1 << (num_generators - 1) before checking whether that exponent even fits in the word. Once num_generators - 1 reaches 64 the shift itself overflows, so the friendly "too many generators" bail never gets to fire. Full walkthrough in the issue.

What that looks like on current main with partition_accounts(130, 65):

• Debug build: panic at accounts.rs:103 with "attempt to shift left with overflow"
• Release build: the shift distance wraps and the input sails through the guard. The partition comes back as Ok with garbage:

[(0, 130), (130, 0), (0, 0), (0, 0), ... 50+ empty ranges ..., (0, 1), (1, 1), (1, 2), (2, 4), ..., (65, 130)]

The first generator gets all 130 accounts, the second gets an inverted range, and the tail ranges all overlap the first one, so generators collide on the same accounts and nonces.

Fix

Short-circuit the guard on the shift width. Once the exponent reaches the word size, 2^(num_generators - 1) already exceeds any possible num_accounts, so the smallest bucket is empty and bailing is the right answer anyway:

let shift = num_generators - 1;
if shift >= usize::BITS as usize || round_div(num_accounts, 1 << shift) == 0 {
    eyre::bail!("too many generators: it would result in a bucket with size 0");
}

This also keeps the boundary loop below safe, since it can no longer be reached with an out-of-range exponent.

Testing

• Added a regression test next to the existing partition tests covering (130, 65) and (200, 100), both now return the same graceful error as smaller over-subscriptions like (100, 9)
• The repro panics on main and passes with this change, existing partition tests still pass
• fmt and clippy clean