docs(signal/cir): precise diagnosis of remaining ADR-134 P2 rms-spread failure

Diagnosed the one still-ignored CIR test: ISTA emits a spurious ~15-20%-of-
dominant tap at an implausible far delay (~bin 150 / ~3us) that inflates
rms_delay_spread to ~390ns (vs ~53ns true). It sits too close to the real
weakest tap (~30% of dominant) for a safe magnitude cutoff, so the proper fix
is ISTA recovery-quality work (grid de-aliasing / far-tap suppression), not a
band-aid threshold. Sharpened the #[ignore] note accordingly. signal+cir:
470 pass / 0 fail.

Co-Authored-By: claude-flow <ruv@ruv.net>