forgefirm-image: install python3-asyncio

The release image carries only the standard-library module packages its own
Python recipes declare, and none of them imports asyncio, so it was left
out. An extension runs on the image's interpreter and brings no
standard-library module of its own: the set it may import is the set
installed. Its event-stream and socket clients are written on asyncio, so
the image installs python3-asyncio on purpose, with the reason in the
recipe. The other modules the trim leaves out (multiprocessing, xmlrpc, the
debugger, venv, ensurepip) stay out until something asks for them.

Proven: an image pair built from this recipe has
/usr/lib/python3.12/asyncio/__init__.py in the release rootfs, read out of
the ext4 image; on the bench reference, running the dev image of that pair,
python3 -c "import asyncio" succeeds.

Acceptance: the change is recipe body, so it is platform content and takes
the full campaign. No catalog test can hold the module itself: forgetest
runs on the dev image, which installs python3-asyncio with the rest of the
standard library whatever this recipe says, so the release rootfs is the
thing to read, as it was here.

meta-forgefirm/recipes-forgefirm/images/forgefirm-image.bb

@@ -18,12 +18,19 @@ DESCRIPTION = "OpenGlow/ForgeFIRM image for Glowforge"
 # removal spec is expanded when applied, so the variable, not the removal,
 # is what a requiring recipe can override). python3 is the meta-package:
 # it installs every standard-library module (tkinter, idle, 2to3, pydoc,
-# ensurepip, venv, the debugger, asyncio, multiprocessing, xmlrpc), which
-# nothing here imports. Each Python recipe declares the module packages it
+# ensurepip, venv, the debugger, multiprocessing, xmlrpc), which nothing
+# here imports. Each Python recipe declares the module packages it
 # imports (python3-core plus the few it uses), so the meta-package goes.
 FORGEFIRM_RELEASE_TRIM ?= "nano"
 IMAGE_INSTALL:remove = "python3 ${FORGEFIRM_RELEASE_TRIM}"
 
+# The Python an extension can count on. An extension runs on the image's
+# interpreter and brings no standard-library module of its own, so the set
+# it may import is the set installed here: what ForgeFIRM's own Python
+# recipes pull in, plus python3-asyncio, which no ForgeFIRM program imports
+# and which an extension's event-stream and socket clients are written on.
+IMAGE_INSTALL:append = " python3-asyncio"
+
 # grblhal-glowforge: the grblHAL motion controller (Grbl over TCP:23).
 # forgectrl: the ForgeFIRM machine-services daemon (HTTP :80, HTTPS :443
 # with a self-signed certificate): controller supervisor, pulse-device