docs(mch2022): explain why a Hatchery app's binary must be main.bin

[annejan]

Closes #95.

The issue asked whether an AppFS app really has to be called main.bin, and
noted that "if that turns out to be true it should be documented". It is true,
but not for the reason the issue guessed.

The answer

AppFS does not care about file names. It stores apps under a slug, and
keeps the file name only as a title.

The badge firmware does care. Installing an ESP32 app from the Hatchery
walks the app's file list and writes exactly one file into AppFS: the one whose
name equals the esp32_bin_fn constant, "main.bin". Every other file is
downloaded into the app's folder as a data file. The comparison is a plain
strcmp in
main/app_management.c
and
main/menus/hatchery.c.
So an app whose binary is myapp.bin installs without an error and then has
nothing to run — which matches the IRC report in the issue.

FPGA apps work the same way: the launcher opens bitstream.bin in the app
folder and looks nowhere else
(main/menus/launcher.c).

Pushing over USB is unaffected. app_push.py <file> <name> <title> <version> takes the AppFS name as an argument, so the local file name is
irrelevant and an update matches on the name, not the file. No change is needed
in the tools or the Hatchery, which is the other half of what the issue asked.

The change

A "Why it has to be called main.bin" subsection on the Hatchery page, with
the above, the app_push.py example, and links to the three source files. The
FPGA paragraph gets the same one-line reason.

hugo --gc --minify clean; all three source links return 200.

🤖 Generated with Claude Code

https://claude.ai/code/session_0142DuVFXpWnjeQhZzT3N3nC