The C assembler names its output for the format, not always .bin

It always wrote .bin, whatever it had built. So assembling a loadable
program without -o produced Say.bin containing SBEX - a boot image name on a
file the machine cannot boot, in a repository whose whole convention is that
a .bin is started from and a .sbx is loaded.

    Successfully wrote SplitBit boot image to "hello.bin".
    Successfully wrote SplitBit loadable program to "Say.sbx".

programIsLoadable() already existed and is already what decides which
writer runs; the name now asks it too. Nothing in the build depended on the
old behaviour, because everything that assembles anything passes -o.

THE ASSEMBLER THAT RUNS ON SPLITBIT ALREADY DID IT THIS WAY. Two assemblers
naming their output differently from the same source is exactly the kind of
difference that wastes an afternoon, and the newer one was right.

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

SplitBit Assembler Manual.md

@@ -417,7 +417,7 @@ Assembler [options] <sourcefile>
 
 | Option | Meaning |
 | -- | -- |
-| -o, --output \<file\> | Write the output to this path. Without it, the output is named after the source file, with a .bin extension, in the directory the assembler was run from. |
+| -o, --output \<file\> | Write the output to this path. Without it, the output is named after the source file, in the directory the assembler was run from, taking .bin if it is a boot image and .sbx if it is a loadable program. |
 | -I, --include \<dir\> | Look in this directory for included files. May be given more than once, and the directories are searched in the order given. |
 | -M, --depend \<file\> | Write out which source files went into the output, as a make rule. |
 | -h, --help | Print the options and stop. |