Mirror the source tree onto the system disk

A list of files in a makefile goes stale the moment somebody adds a program and forgets to
name it, and what they forgot is invisible until they go looking for it on the machine. So
SplitDisk gained a mirror command and the disk rule is one line: putting a file where the
others live is now the whole of putting it on the disk.

EVERY FILE GOES THROUGH put AND EVERY DIRECTORY THROUGH mkdir. That is the point of it -
mirror adds a walk and no filesystem code at all, so anything the format refuses here it
refuses everywhere, in the same words. What is new is the walk, and the walk is what the
six checks in Tests/disk.sh are about: that it goes all the way down, that it leaves dotfiles
and named directories behind, and that a name too long stops it.

REFUSED RATHER THAN SKIPPED, because a disk quietly missing a file is the exact failure a
mirror exists to prevent. Which meant four sources had to be renamed - a directory entry
holds 22 characters and they were 23, 23, 24 and 29:

  16bitSegmentedSieve.asm        -> 16bitSieve.asm
  16bitSegmentedSieveModern.asm  -> 16bitSieveModern.asm
  consoleInterruptTest.asm       -> consoleInterrupt.asm
  controllerWriteTest.asm        -> controllerWrite.asm

The test names in the manifest are unchanged, since those are identifiers and every recorded
result is filed under them. Only where the source lives has moved.

The entries are sorted before anything is written. readdir hands them back in whatever order
the host filesystem feels like, and a disk image that comes out different from one run to the
next is an image no test could compare against another.

The disk grew from one megabyte to four and from 192 directory entries to 1,024. The sources
are 2,850 blocks and the mirror filled the old directory on its first run, which is a thing
that should not need thinking about again.

The Tests fixture disk is deliberately NOT mirrored. It is a controlled fixture with known
contents, and the shipped disk is the one meant to be useful; they want different things.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Programs/CosmOS/README.md

@@ -93,6 +93,31 @@ The generated files are kept under `Programs/build/`:
 - `CosmOS/Apps/*.sbx` are loadable application images.
 - `cosmos.img` is the SBFS disk containing those applications.
 
+**The disk carries every source in `Programs/`, mirrored.** Not a list kept in the makefile -
+a list goes stale the moment somebody adds a program and forgets to name it, and what they
+forgot is invisible until they go looking for it on the machine. Putting a file where the
+others live is the whole of putting it on the disk.
+
+That matters most for the things nobody thought worth shipping a binary of. A demo that is
+not interesting enough to build by default is still worth having the source of, because the
+machine can build it:
+
+```
+> cd /Source/Examples
+/Source/Examples> Asm colours.asm
+wrote colours.bin: program 114, data 86, labels 8
+```
+
+Two things are left behind. `build`, because what a project builds is not what it wrote. And
+anything whose name is longer than a directory entry holds, which is **refused rather than
+skipped**: a disk quietly missing a file is exactly the failure a mirror exists to prevent,
+so the build stops and says which name to shorten.
+
+`/Lib` still holds the library sources separately, because that is where an `#Include` looks
+after looking beside the file that asked. The same files therefore appear twice - once as
+what a program includes, once as part of the source tree - and that is the difference between
+an installed library and a copy of the source.
+
 The disk is rebuilt from scratch when its applications change, so its contents describe
 the current source tree rather than accumulating files left by older builds.