Describe the length of Play sound and the idle easing rows

- Sounds: Play sound hands out length, and a point says how to span
  the whole sound with it and why a pitch change does not end there.
- Behaviors and Sounds: as on a key, Direction and Curve only show
  where they matter, on the node and in the inspector.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Behaviors.md

@@ -226,7 +226,8 @@ the inspector).
   *Easing* and *Direction* bend the way with exactly the choice an
   [animation key](Animations) has: *Linear*, *Step* (stays at a until t is 1), the ten curves from
   *Sine* to *Bounce* with *In*, *Out* or *In-out*, or *Curve*, which follows the
-  [curve asset](Curves) picked in the *Curve* row. *Back*, *Elastic* and a curve may overshoot b;
+  [curve asset](Curves) picked in the *Curve* row. As on a key, *Direction* only shows for one of
+  the ten curves and the *Curve* row only for *Curve*, on the node and in the inspector. *Back*, *Elastic* and a curve may overshoot b;
   a Curve with none picked, or once it is gone, runs linear and says so on the node —
   one node where a slide or a flight used to take a Subtract, a Multiply and an Add per axis),
   `Floor/Ceil/Round`, `Modulo`, `Random`, `Time`, `To text`, `To number`, `Join text`,

Sounds.md

@@ -36,7 +36,7 @@ so what the inspector plays is what the graph will play.
 
 | Node | What it does |
 | --- | --- |
-| **Play sound** | Starts the sound named by its `sound` input. `volume` **multiplies** the asset's own volume (1 leaves it alone), `loop` is a choice of three (*From the asset*, *On*, *Off*), `detached` says the voice belongs to nobody, `fx` names a [sound FX](#sound-fx) to play it through, and `fx params` takes values for that FX's [parameters](#parameters). Its output `voice` is the number of this one play, for changing it [while it plays](#changing-a-playing-sound); 0 where nothing was played. |
+| **Play sound** | Starts the sound named by its `sound` input. `volume` **multiplies** the asset's own volume (1 leaves it alone), `loop` is a choice of three (*From the asset*, *On*, *Off*), `detached` says the voice belongs to nobody, `fx` names a [sound FX](#sound-fx) to play it through, and `fx params` takes values for that FX's [parameters](#parameters). Its output `voice` is the number of this one play, for changing it [while it plays](#changing-a-playing-sound), and `length` how many seconds one run of the sound lasts; both 0 where nothing was played. |
 | **Stop sound** | Silences every voice of the sound it names, however many plays started it — a loop included. Naming no sound silences everything the run is playing. |
 | **Set sound FX parameters** | Hands new values to the [parameters](#changing-a-playing-sound) of one voice that is already playing, by the `voice` number its `Play sound` gave out, at once or over `time` seconds. |
 | **Sound** | A constant sound, for either of the two to work on through an edge. |
@@ -178,7 +178,8 @@ speed, or a loop that gets quieter.
   dB, a band and a Mix in what they hold. It runs on the clock of the sound itself, not on the
   game's, so a sweep sounds the same every time, also the first.
 - **Easing** and **Direction** say how the values get there, with the same choice an
-  [animation key](Animations) and a `Lerp` have. *Linear* (the default) runs evenly, *Step* holds
+  [animation key](Animations) and a `Lerp` have, and like there *Direction* and *Curve* only show
+  where they matter. *Linear* (the default) runs evenly, *Step* holds
   the old value for the whole `time` and then jumps, the ten curves from *Sine* to *Bounce* and a
   drawn [Curve](Curves) bend the way. Linear and Step are ramps of the audio clock itself, every
   other easing is handed to it as a curve of 200 samples a second (`setValueCurveAtTime`). *Back*,
@@ -187,6 +188,12 @@ speed, or a loop that gets quieter.
   right after `Play sound`: `On enable` → `Play sound` → `Set sound FX parameters` (`voice` from
   `Play sound`, `pitch` -64, `time` 1.28) falls by 50 semitones a second. Counting a value down
   in `On update` or `On fixed update` instead runs on game time, and the sound drifts away from it.
+- **To span the whole sound**, wire `length` of `Play sound` into `time`: a fade or a filter sweep
+  then ends with the last sample. `length` is one run of the sound (one pass of a loop), the same
+  with a pitch it starts on. It is 0 while the sound has not loaded yet, which only happens right
+  after a project is opened, and `time` 0 is the quick glide. A pitch changed over `length` does
+  not end with the sound: a lower pitch plays slower (see the next point), so the rest of the sound
+  is heard at the target pitch, and a higher one ends before it gets there.
 - A new change starts from where the voice is right now; whatever was still under way is dropped.
 - **A pitch changes by speed here.** A pitched copy cannot be worked out again in the middle of a
   sound, so the voice plays faster or slower to reach the new pitch: it gets that much shorter or