src/filters.c: Add SMULR64 speedup to parametric_eq_process.

[dpwe]

Adds speedup from #951 to EQ, even bigger win.

[github-actions]

🎛️ AMY HW CI (AMYboard bench)

Flashed this PR's AMY (LoadTestChord: 6-voice Juno patch=1, one held note every 2 s) onto the physical AMYboard and measured the smoothed render load as the chord grows — back-to-back with the same sketch built at the PR's merge base, so Δ is this PR's own cost.

✅ PASS — the bench ran the test to completion.

notes held	main @ 2a1745e	this PR	Δ
1	1382	1021	-361
2	1544	1185	-359
3	2134	1749	-385
4	2322	1931	-391
5	2973	2553	-420
6	3172	2747	-425

Full chord settled render μs: 2746 (was 3172, Δ -13.4%) (peak 2754, 39 samples)

⬇️ Artifacts: serial log · load trace · report

_{Self-hosted bench (amyboardci). FAIL means only that the test could not run — the load values are informational, with no threshold and no audio compare. See tools/arduino_loadsweep/.}