Feat: adding LPG

docs/engine/audio_modulation.md

@@ -57,17 +57,53 @@ saw snd 200 4000 1 drunk lpf .       ( random walk, each step )
 
 Stack effect: `( min max period -- str )`
 
-## Envelopes
+## Envelope Modulation
 
-Define a multi-segment envelope for a parameter. Provide a start value, then pairs of target and duration.
+Apply an envelope to any parameter. The `env` word is the complete form: it sweeps from `min` to `max` following a full attack, decay, sustain, release shape. All times are in steps.
 
 ```forth
-saw snd 0 1 0.1 0.7 0.5 0 8 env gain .
+saw snd 200 8000 0.01 0.1 0.5 0.3 env lpf .
 ```
 
-This creates: start at `0`, rise to `1` in `0.1` steps, drop to `0.7` in `0.5` steps, fall to `0` in `8` steps.
+Stack effect: `( min max attack decay sustain release -- str )`
 
-Stack effect: `( start target1 dur1 [target2 dur2 ...] -- str )`
+This is the building block. From it, three shorthands drop the parameters you don't need:
+
+| Word | Stack | What it does |
+|------|-------|-------------|
+| `env` | `( min max a d s r -- str )` | Full envelope (attack, decay, sustain, release) |
+| `eadr` | `( min max a d r -- str )` | No sustain (sustain = 0) |
+| `ead` | `( min max a d -- str )` | Percussive (sustain = 0, release = 0) |
+
+`eadsr` is an alias for `env`.
+
+```forth
+saw snd 200 8000 0.01 0.3 ead lpf .              ( percussive filter pluck )
+saw snd 0 5 0.01 0.1 0.3 eadr fm .               ( FM depth with release tail )
+saw snd 200 8000 0.01 0.1 0.5 0.3 env lpf .      ( full ADSR on filter )
+```
+
+These work on any parameter — `lpf`, `fm`, `gain`, `pan`, `freq`, anything that accepts a value.
+
+## Low Pass Gate
+
+The `lpg` word couples the amplitude envelope with a lowpass filter. Set your amp envelope first with `ad` or `adsr`, then `lpg` mirrors it to `lpf`.
+
+```forth
+saw snd 0.01 0.1 ad 200 8000 1 lpg .        ( percussive LPG )
+saw snd 0.01 0.1 0.5 0.3 adsr 200 4000 1 lpg .  ( sustained LPG )
+```
+
+Stack effect: `( min max depth -- )`
+
+- `min`/`max` — filter frequency range in Hz
+- `depth` — 0 to 1, scales the filter range (1 = full, 0.5 = halfway)
+
+```forth
+saw snd 0.01 0.5 ad 200 8000 0.3 lpg .      ( subtle LPG, filter barely opens )
+```
+
+`lpg` reads `attack`, `decay`, `sustain`, and `release` from the current sound. If none are set, it defaults to a short percussive shape.
 
 ## Combining
 
@@ -77,6 +113,6 @@ Modulation words return strings, so they compose naturally with the rest of the
 saw snd
   200 4000 4 lfo lpf
   0.3 0.7 8 tlfo pan
-  0 1 0.1 0.7 0.5 0 8 env gain
+  0 1 0.01 0.1 ead gain
 .
 ```