docs(ideas): record design checks for Anvil Ghost harmonics

docs/ideas.md

@@ -221,7 +221,18 @@ one period of the note, so heavy settings do not ripple at f0. See [Planish](pla
   of the sub sine above a cutoff you set, with weights that follow the
   patch's own harmonic slope, so phones hear the note; separate from the sub
   level, so the physical sub does not change (problem 19). A *small speaker*
-  listen button that band-limits the output to judge it.
+  listen button that band-limits the output to judge it. Design checks before
+  building Ghost: Anvil's Drive already adds harmonics without
+  intermodulation (one sine through tanh), but only odd ones and it changes
+  the fundamental; harmonics 2–5 of a 50 Hz sub (100–250 Hz) still sit under a
+  phone's cutoff (often 300–500 Hz), so Ghost needs a "from" frequency and
+  harmonics up to about 10; and generated harmonics at k·f0 add to the
+  patch's own harmonics at the same frequencies with unrelated phase, so they
+  can cancel. The paper boosts harmonics that exist and generates only missing
+  ones: Lathe already boosts existing ones, so Ghost should fill only
+  harmonics the patch lacks above the split (measure the high band per
+  harmonic, as `common-dsp` `meter.rs` does). Most useful on 808s and clean
+  subs; a riddim patch already carries its note in the mids.
 - **Lathe**: A/B gain snapshots with one automatable Morph parameter. A
   keyswitch note that selects the drawn shape (EQ switch-ups without
   automation). A dynamic mode per harmonic.