Describe a fade in and out, each along its own curve (#345)

- Sounds: Fade in, Fade out and a curve for each replace Edge fade,
  in the table, the Sound tab's header and the cut's name. A new
  section, The curve of a fade, shows all five curves.
- Development: 1852 editor checks.

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

Development.md

@@ -19,7 +19,7 @@ php tests/php/accounts.test.php                               # the API's accoun
 php tests/php/snapshots.test.php                              # the snapshot store (#231), the same way
 php tests/php/releases.test.php                               # the release store (#266), the same way
 php tests/php/files.test.php                                  # uploaded files in a folder per account (#344), the same way
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1850 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1852 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result

Sounds.md

@@ -31,7 +31,8 @@ signature, so nothing a request claims can decide what is written.
 | **Volume** | How loud this sound is by itself, 0-1. |
 | **Loop** | Whether it is music: it repeats until something stops it. |
 | **Start (s)**, **End (s)** | The stretch of the file the sound is, set in its [tab](#the-sound-tab). End 0 is the end of the file. |
-| **Edge fade (ms)** | How long each cut edge inside the file fades in or out, 10 by default. |
+| **Fade in (ms)**, **Fade out (ms)** | How long the start fades in and the end fades out when that edge lies inside the file, 5 each by default (#345). |
+| **Fade in curve**, **Fade out curve** | The shape of each fade: *Linear* (the default), *Equal power*, *S-curve*, *Exponential* or *Logarithmic*. See [below](#the-curve-of-a-fade). |
 
 The preview plays through the very same library a running game uses — same decode, same device —
 so what the inspector plays is what the graph will play, cut and all.
@@ -48,11 +49,14 @@ waveform, a Sample's loop points, a Pitch.
 - **Drag a handle** to move that edge. **Drag anywhere else** to span a new stretch from where you
   pressed. One drag is one undo step, the two edges together. An edge cannot pass the other.
 - **Whole file** in the header puts both handles back on the ends of the file.
-- **Edge fade (ms)** in the header (also in the inspector) is the ramp at each edge **inside**
-  the file: the stretch rises from silence over it and falls back to it, so a cut through a wave
-  does not click. It is counted into the stretch, so the stretch stays as long as it is. An edge
-  on the start or the end of the file is left as it is. A fade longer than half the stretch is
-  shortened to half. The ramps are drawn in the wave.
+- **Fade in (ms)** and **out** in the header (also in the inspector) are the ramps at the two
+  edges **inside** the file: the stretch rises from silence over the one and falls back to it
+  over the other, so a cut through a wave does not click. They are counted into the stretch, so
+  the stretch stays as long as it is. An edge on the start or the end of the file is left as it
+  is. A fade longer than half the stretch is shortened to half. The dropdown behind each field is
+  the **curve** of that fade. It shows the curve as a small picture, rising for the fade in and
+  falling for the fade out; its tooltip names it. The ramps are drawn in the wave, each in its
+  curve.
 - **Play** (or Space) plays the stretch once, at the asset's volume, with a playhead. It ends
   whatever pattern or song the music tabs were playing, as they end each other.
 - The **wheel** zooms about the pointer, the **middle button** pans, as in the music tabs. Zoomed
@@ -64,9 +68,9 @@ waveform, a Sample's loop points, a Pitch.
 at any time, and **two cuts of one file** are two assets (duplicate the asset, pick another
 stretch) that share the file on the server.
 
-**How it plays.** A cut sound is played by a name of its own, `<file>#<start>~<end>~<fade>`. The
-library fetches and decodes the file once and makes the stretch from it the first time that name
-plays: a copy of those samples with the ramps worked in. Length and waveform of a cut are read
+**How it plays.** A cut sound is played by a name of its own,
+`<file>#<start>~<end>~<fadeIn>~<fadeOut>~<curveIn>~<curveOut>`. The library fetches and decodes the file once
+and makes the stretch from it the first time that name plays: a copy of those samples with the ramps worked in. Length and waveform of a cut are read
 from the file itself, without a copy. Copies are made again whenever they are needed, so the
 library lets the oldest go once they hold more than 64 Mi samples together — a cut dragged about
 in its tab leaves no pile of copies behind. A sound over the whole file is no cut at all and plays
@@ -75,6 +79,23 @@ the file as it always did; assets saved before the tab existed are such sounds.
 **Song items** keep their place and their length when their sound is cut anew: they play from the
 new start, and one longer than the new stretch plays it round and round.
 
+### The curve of a fade
+
+A fade runs from silence to full. Its curve says how loud it is on the way, at `t` from 0 (the
+silent edge) to 1 (full). A fade out runs its curve backwards, from full to silence.
+
+| Curve | Loudness at `t` | Halfway | Sounds like |
+| --- | --- | --- | --- |
+| *Linear* | `t` | 0.5 | A straight ramp, the default. |
+| *Equal power* | `sin(t · 90°)` | 0.71 | Keeps the power even. A fade out of one sound and a fade in of the next laid over each other stay as loud as one. |
+| *S-curve* | `(1 - cos(t · 180°)) / 2` | 0.5 | Soft at both ends of the ramp, steep in the middle. |
+| *Exponential* | `t²` | 0.25 | Stays quiet long near the silent edge. A fade out falls quickly and then lingers, like a sound dying away. |
+| *Logarithmic* | `1 - (1 - t)²` | 0.75 | Gets loud quickly. A fade out holds long and drops at the very end. |
+
+Each fade has a curve of its own, so a sound can, say, rise along *Linear* and die away along
+*Exponential*. Sounds saved before #345 had a single *Edge fade*. It is not read anymore, so they
+start with 5 ms each and *Linear* on both.
+
 ## Playing it
 
 | Node | What it does |