fix(spammer): reject num_generators that would overflow the partition shift

[Kewe63]

Summary

PartitionMode::Exponential::partition_exponential in crates/spammer/src/accounts.rs performed two unchecked left shifts on num_generators. On a 64-bit target, num_generators >= 65 makes the shift distance reach usize::BITS and the << operation overflows:

• Debug builds — panics with attempt to shift left with overflow
• Release builds — shift distance silently wraps; 1usize << 64 evaluates to 1usize << 0 == 1, the smallest-bucket guard sees a non-zero bucket and lets the input through, and the boundary loop emits 64 zero-length ranges plus a final correct range

Verified locally on upstream/main (a85368c) before applying the fix.

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Root Cause

if round_div(num_accounts, 1 << (num_generators - 1)) == 0 { ... }

for j in (1..=num_generators - 1).rev() {
    let denom = 1usize << j;
    ...
}

partition_exponential already shifts by num_generators - 1, so the natural upper bound is usize::BITS — a left shift by usize::BITS overflows on every platform, and beyond that the partition concept breaks down (1 << 63 already gives sub-unit smallest buckets for any realistic num_accounts).

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Fix

Reject any num_generators > usize::BITS up front with a clear error message, before either shift is computed.

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Changes

File: crates/spammer/src/accounts.rs

• Added early validation rejecting num_generators > usize::BITS with an explicit error.
• Added test partition_exponential_rejects_shift_overflow covering:
  • Original 65-generator case
  • usize::BITS + 1 boundary
  • usize::MAX extreme

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How to Test

# Debug build
cargo test -p spammer --lib
# ✅ 65 passed, 0 failed

# Release build
cargo test -p spammer --lib --release
# ✅ 65 passed, 0 failed

# Format & lint
cargo fmt --check -p spammer
cargo clippy -p spammer --all-targets -- -D warnings
# ✅ Both clean

Pre-existing partition_accounts_linear and partition_accounts_exponential tests are unchanged.

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Risk & Impact

Low. Validation is purely additive — valid inputs (num_generators <= usize::BITS) are unaffected. Inputs that previously caused silent UB in release builds or panics in debug builds now return a clear error.

Type: 🐛 Bug fix
Closes: #137