perf: replace heap allocations with stack arrays in logup inner loops

[jiayaoqijia]

Fixes #154.

Replaces two Vec heap allocations with stack-allocated fixed-size arrays in the hot inner loops of prove_generic_logup:

1. Bytecode lookup denominator (line 119): Vec::with_capacity(1 + N_INSTRUCTION_COLUMNS) → [F::ZERO; N_INSTRUCTION_COLUMNS + 1] — matching the existing pattern at lines 88-90.

2. Bus denominator (lines 147-153): .collect::<Vec<_>>() → [F::ZERO; N_INSTRUCTION_COLUMNS] with explicit length slice &bus_data[..bus.data.len()]. All bus data lengths are ≤ 4 (verified across all table implementations), well within the N_INSTRUCTION_COLUMNS = 12 bound.

3. Bytecode columns collection (line 111): .collect::<Vec<_>>() → slice reference, avoiding an unnecessary Vec allocation.

These allocations occur inside par_iter_mut().for_each() closures, meaning they execute once per row of the trace — potentially millions of times. Stack arrays eliminate per-iteration allocator pressure and improve cache locality.