perf(stark): skip fixed 0/1 muls in LogUp fingerprint accumulation

[diegokingston]

In the fingerprint hot loop (prover aux-build + constraint-eval + verifier):

• Bus-id term: alpha_powers[0] = alpha^0 = 1, so embed the bus id into the extension field directly instead of multiplying by 1 (drops one F*E mul per interaction per row, hoisted out of the row loop on the aux path).
• Fixed-zero bus elements (the ~235 constant(0) used for bus-width padding) contribute nothing: skip the F*E multiply + accumulate entirely. Variable elements that happen to be zero on a row also benefit.

Value-identical (field addition is exactly associative): stark lib 128/128 (default + parallel), prover bus/logup tests pass, clippy clean. Net effect on prove time is what we want to measure on the 32-core bench.

[diegokingston]

/bench 5

[github-actions]

Benchmark — fib_iterative_8M (median of 5)

_{Table parallelism: auto (cores / 3)}

Metric	main	PR	Δ
Peak heap	34843 MB	34815 MB	-28 MB (-0.1%) ⚪
Prove time	18.689s	18.473s	-0.216s (-1.2%) ⚪

✅ No significant change.

✅ Low variance (time: 2.5%, heap: 1.1%)

_{Commit: 974eaad · Baseline: cached · Runner: self-hosted bench}