Describe the sound FX of its own in a song (#355)

- Songs: new section "A sound FX of its own" and a note at the FX
  button of a track head.
- Songs, Not yet: a pattern item can get a pattern of its own since
  Embed a copy, the old sentence said otherwise.
- Development: 1889 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                 # 1887 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1889 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

Songs.md

@@ -55,7 +55,7 @@ the right: it starts that far before the end of a round.
 ## The Song tab
 
 Down the left are the **heads** of the tracks: the name, **FX** (the track's sound FX, blue when it
-has one), **M** (mute, orange when on) and **S** (solo, yellow when on), below them the volume (the line marks 0 dB) and the pan (the line marks the
+has one; its menu can also make [one of its own](#a-sound-fx-of-its-own)), **M** (mute, orange when on) and **S** (solo, yellow when on), below them the volume (the line marks 0 dB) and the pan (the line marks the
 middle). A track is added from the right-click menu of a head, above or below it. Along the top is the **ruler**, as in the
 Pattern tab. Between them are the **lanes** with the items: a pattern blue with its notes drawn
 small, every round it plays; a sound green with its waveform, from where it starts in it, every
@@ -249,6 +249,21 @@ song) and an item for it, one undo step, and opens it in the Pattern tab.
 - A project pattern embedded as a copy, or one pasted from another song, takes the clock of the
   song it comes to. Made a project asset, it keeps the tempo and the meter it had.
 
+### A sound FX of its own
+
+Every pick of a sound FX in a song ends with a few entries for one that only that place has: the
+**Sound FX** row of the song, of a track and of an item, and the menu of a track's **FX** button.
+
+- **Create sound FX** makes *Sound FX 1*, 2 … (numbered per song) with its Input wired to its
+  Output, so it lets the sound through until something is put in between. The place picks it and
+  the Sound FX tab opens it, all in one undo step.
+- **Embed a copy**, while the place names a sound FX of the project: the place gets a copy of it,
+  under the same name, to bend without touching the original.
+- While the place names its own, the menu shows it with the mark in front and offers **Edit sound
+  FX** (opens it, as a double-click on its name in the inspector does) and **Make project asset**.
+- Picking anything else, *None* or another sound FX, lets it go in the same undo step. That holds
+  for the song's own row too.
+
 ## Playing one in a game
 
 **Play song** plays a song at its tempo, its tracks with the volume, pan, mute and solo they have
@@ -319,5 +334,6 @@ bus. Stop let it go. An item's *Sound FX* offered *From the asset*, *None* and t
 
 A link sends at one volume; it cannot be moved over time, and there is no reverb to send to yet
 (#328). Nothing tells a graph that a song played once has ended, and there is no node that changes the
-volume of a playing song. No start at a marker (#338), no automation (#337), no song inside a song, no held note (drone) as an item. A pattern item cannot be made into a pattern of its own (*make unique*); for a
-variation, duplicate the pattern in the Assets panel and pick it on the item.
+volume of a playing song. No start at a marker (#338), no automation (#337), no song inside a song, no held note (drone) as an item. A variation of a pattern for one item is *Embed a copy* in the
+item's menu (see [Embedded assets](#embedded-assets)). A bus cannot have a sound FX of its own,
+and a song cannot keep an instrument to itself.