Say what the tests need, and count the tools correctly

Two nitpicks from review, and one of them was only half right, which is why
each was checked rather than swept.

THE TEST DEPENDENCIES were nowhere. The README said what building needs and
stopped, so somebody with a C compiler and nothing else would get through
'make' and fall over on 'make test'. It needs bash and Python 3 - two of the
checking scripts are Python, and one of those opens a pseudo-terminal - plus
stty, timeout and the usual text utilities.

"BOTH TOOLS" WAS WRONG IN TWO PLACES OUT OF SIX. 'make sanitize' builds the
default target, which is all three, so the README and the makefile both
undercounted what they rebuild.

The other four are right and were left alone. assembly.h says both tools
have to agree where the vector table begins, and bootstrap.c and cpu.c say
they share the boot image format and the vector layout - in all three cases
that is the emulator and the assembler, and SplitDisk has no opinion about
any of it. The makefile's POSIX comment is the same story: SplitDisk uses
not one POSIX interface, so it names the two that do rather than counting to
three.

Media/ is a home for the screenshot that is coming, with a note saying what
belongs in it and that docs.sh will catch a link to something removed from
it.

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

makefile

@@ -11,7 +11,8 @@ PREFIX ?= /usr/local
 # editing a header rebuilds everything that includes it.
 DEPFLAGS = -MMD -MP
 
-# Both tools use POSIX interfaces that ISO C does not have: realpath, clock_gettime,
+# The emulator and the assembler use POSIX interfaces that ISO C does not have:
+# realpath, clock_gettime,
 # strdup, dirname and getopt. Asking for POSIX.1-2008 by name means the build does
 # not rely on the compiler happening to default to a mode where those are visible,
 # and it survives someone overriding CFLAGS, which is why it is kept separate.
@@ -104,7 +105,7 @@ test: $(EMU_TARGET) $(ASM_TARGET) $(DSK_TARGET) strict
 	@echo
 	@./Tests/docs.sh
 
-# Rebuild both tools with the address and undefined behaviour sanitizers and run
+# Rebuild all three tools with the address and undefined behaviour sanitizers and run
 # the test suite under them. Slower than 'make test', and worth running before a
 # release or after anything that touches memory handling.
 #