Feat: document time stretching

docs/engine/samples.md

@@ -50,6 +50,7 @@ snare sound  0.5 speed .         ( play snare at half speed )
 | `slice` | 1+ | Divide sample into N equal slices |
 | `pick` | 0+ | Select which slice to play (0-indexed, wraps) |
 | `speed` | any | Playback speed multiplier |
+| `stretch` | 0+ | Time-stretch factor (pitch-independent) |
 | `freq` | Hz | Base frequency for pitch tracking |
 | `fit` | seconds | Stretch/compress sample to fit duration |
 | `cut` | 0+ | Choke group |
@@ -105,6 +106,24 @@ crow sound -1 speed .              ( play backwards at nominal speed )
 crow sound -4 speed .              ( play backwards, 4 times faster )
 ```
 
+## Time Stretching
+
+The `stretch` parameter changes sample duration without affecting pitch, using a phase vocoder algorithm. This contrasts with `speed`, which changes both tempo and pitch together.
+
+```forth
+kick sound 2 stretch .             ( twice as long, same pitch )
+kick sound 0.5 stretch .           ( half as long, same pitch )
+kick sound 0 stretch .             ( freeze — holds at current position )
+```
+
+Combine with `slice` and `pick` for pitch-locked breakbeat manipulation:
+
+```forth
+break sound 8 slice step pick 2 stretch .   ( sliced break, stretched x2, original pitch )
+```
+
+Reverse playback is not available with `stretch` — use `speed` for that.
+
 ## Fitting to Duration
 
 The `fit` parameter stretches or compresses a sample to match a target duration in seconds. This adjusts speed automatically.