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/main.ludic

@@ -51,7 +51,7 @@ program Ludic {
     print("  version                       print the toolchain version")
     print("  upgrade [version]             reinstall from the docs site (the same script that installed it)")
     print("  doctor                        check that the install is complete and usable")
-    print("  fmt [paths...]                format Ludic source in place")
+    print("  fmt [--check] [paths...]      format Ludic source in place (--check: report, write nothing)")
     print("  lsp                           run the language server on stdio (what editors spawn)")
     print("  help                          this message")
     print("")