fix(cli): scaffold a project that compiles, and validate flags

`ludic new my-game` wrote `program My-Game`, so the first thing anyone did with
a new project — run it — failed with `expected '{', got '-'`. The project name
is a directory name and the identifier is Ludic source, and they do not accept
the same characters: names are now folded into a valid identifier (my-game ->
MyGame, 2048 -> Game2048, a.b.c -> ABC) and a name that cannot be a directory or
a package is refused with the rule instead of being mangled into one.

An audit of every command's flags turned up more of the same shape, all fixed:

- build/run ignored unknown options, so `--headles` silently produced a windowed
  binary, and `-o` with no path was silently dropped.
- `ludic fmt` printed the formatted text to stdout while its help said "in
  place", so it appeared to do nothing. It writes now, with --check for the
  report-only case a hook wants.
- `ludic test nosuch.ludic` deferred the error to the compiler.
- build-lib's failure messages ran `{tmp_dir()}` through the shell literally —
  an interpolation written inside a non-interpolating string — and its argument
  guess matched package.lock.ludic, then tried to compile the lockfile.
- Several messages still identified the tool as `x`.

`ludic-dev test` now scaffolds under four awkward names, builds and tests each,
and asserts a space-bearing name is refused — the case that shipped broken.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/ludic-cli/selfhost.ludic

@@ -97,7 +97,7 @@ function line_count(path: pointer) -> pointer {
 function cmd_selfhost_build(lc: pointer, outbin: pointer) -> int {
   run("mkdir -p build")
   let src = "build/selfhost.ludic"
-  if not write_selfhost_src(src) { err("x: cannot write build/selfhost.ludic\n"); return 1 }
+  if not write_selfhost_src(src) { err("ludic-dev: cannot write build/selfhost.ludic\n"); return 1 }
   let ll = `{outbin}.ll`
   if not shq(`{lc} {src} > {ll} 2>/dev/null`) { run(`rm -f {ll}`); return 1 }
   let rc = shq(`{cc()} {ll} -o {outbin} 2>/dev/null`)