Make main! optional under roc test

[rtfeldman]

It is common to have a .roc file that exists purely for tests — a bunch of top-level expects meant to be run with roc test foo.roc, which is how Exercism programs are laid out, for example. Until now such a file still had to define main! = |_| Ok({}), even though nothing ever calls it. Beyond being a papercut, that carries a real hazard: the file looks runnable, so a CI script that says roc foo.roc or roc build foo.roc by mistake can appear to succeed without ever having run the tests. roc test now requires no entrypoint at all: a headerless file that is neither a type module nor a default app is compiled as a plain module, and its expects run as usual.

The main! requirement only ever came from canonicalization. A headerless file starts out as a candidate type module; if it has a valid one-argument main! it becomes a default app, and otherwise validateForChecking demands either a nominal type matching the file name or a main!, reporting default_app_missing_main or type_module_missing_matching_type. Nothing in the roc test pipeline itself wants a main! — the expects are collected as compile-time roots on their own — so the diagnostic was the entire obstacle, and it made roc test exit non-zero even while printing that every test passed.

The fix reuses the validation that --main entry modules already got, where a headerless file that resolves to neither shape simply becomes a plain module with its top-level types exposed. That mode was selected by a validate_as_explicit_roots bool threaded from BuildEnv through the coordinator to canonicalizeModuleWithSiblings; it is now an explicit Can.Validation enum (.checking / .explicit_roots) owning the dispatch, which also lets the playground drop its private duplicate of the same two-case enum. roc test sets it on the build's root module via the new BuildEnv.setRootValidation.

roc check, roc build, and roc <file> are deliberately untouched and still reject an entrypoint-less file, so the mistaken-CI-invocation case stays loud. Type modules, module-header files, apps with a platform, and default apps with a real main! all behave exactly as before under roc test.