Describe the pattern made in a song (#355)

Songs gets the Create pattern row in the menu table and a section on
the embedded pattern: where the item lands, which instrument it
plays, the song's clock and the hidden rows. Development counts the
two new checks of e2e 65.

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                 # 1885 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1887 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

@@ -89,6 +89,7 @@ right-click menu, not from the inspector.
 | Click a head | Picks its track. |
 | `Shift`-click an item | Adds it to the pick or takes it out. Dragged on with `Shift`, it is added and moved or sized off the grid instead. |
 | Right-click an item | Opens its menu: *Open* opens what it plays (as a double-click does), *Make project asset* or *Embed a copy* (see [Embedded assets](#embedded-assets)), and *Delete* deletes it, or every picked item where it is one of them. |
+| Right-click an empty spot of a track | *Create pattern*: a new pattern the song keeps to itself (see [A pattern made in the song](#a-pattern-made-in-the-song)). |
 | Double-click a pattern item | Opens the pattern in the Pattern tab, with what the song plays around the round clicked into as [ghost notes](Patterns#ghost-notes-from-the-song). |
 | Double-click a sound item | Opens the sound in the Sound tab (#352). |
 | Click **M** or **S** in a head | Mutes or solos the track. |
@@ -234,6 +235,20 @@ its name only has to be free within the song.
   its project asset that only the song has, to bend without touching the rest.
 - A project file whose song is gone keeps such an asset as a project asset.
 
+### A pattern made in the song
+
+*Create pattern* on an empty spot of a track makes the pattern *Pattern 1*, 2 … (numbered per
+song) and an item for it, one undo step, and opens it in the Pattern tab.
+
+- The item starts on the grid line before the spot and is a bar long, and so is the pattern.
+- It plays the instrument of the pattern item on that track that starts last before the spot,
+  else of the first one after it; on a track without one it has none yet.
+- It runs on the song's clock: it takes the tempo and the meter of the song, and follows them
+  when they change. In its inspector the rows *Tempo*, *Beats per bar*, *Beat* and *Bus* are left
+  out, as the song decides all of them. Its length, grid, instrument and sound FX are its own.
+- 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.
+
 ## Playing one in a game
 
 **Play song** plays a song at its tempo, its tracks with the volume, pan, mute and solo they have