schema: ludicc --emit-schema / ludic schema, --check --diagnostics=json, and editor attributes

--emit-schema FILE writes the compiler's resolved view once the program type-checks: every
record (fields, types, defaults as written, docs, places, attributes), every registry with its
entries in their final order after the open-registry merge (key, constant, index, file:line:col
of the entry and of each field value, and which file contributed which keys), every const, and
the zero-argument functions a fn value can name. Deterministic, schema_version 1; the runtime is
left out. `ludic schema [file] [-o FILE]` wraps it.

--check --diagnostics=json prints every error as one JSON array on stdout: the checker's and the
module rules' all, a parse or lowering error as the last. Tokens and nodes now carry a column.

Fields take several @attributes; @Ref(Registry), @OneOf(PREFIX_), @Range(lo, hi), @Unit("..."),
@Asset("..."), @Color on a field and @AppendOnly / @ByKey on a registry change nothing but go into
the schema, and @Ref naming no registry is an error (every one reported). Fixtures:
examples/lang/attributes.ludic, examples/rejected/ref_unknown.ludic, cases in ludic-dev test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/ludic-cli/schema.ludic

@@ -0,0 +1,48 @@
+# ---- ludic schema -------------------------------------------------------------
+# What an editor of a program's data needs, from the compiler's own view: every record with its
+# fields, types, defaults, docs and attributes (@Ref, @Range, ...), every registry with its entries
+# in their final order and where each is written, every const, and the functions a `fn` value can
+# name. The compiler writes it (`ludicc --emit-schema`, selfhost/backend/emit_schema.ludic) once the
+# program has type-checked.
+#
+#   ludic schema [file]              the JSON on stdout
+#   ludic schema [file] -o FILE      into FILE
+#   ludic schema --unsafe ...        a program whose own files write `unsafe`
+function cmd_schema() -> int {
+  var src = ""
+  var dest = ""
+  var ai = 2
+  while ai < arg_count() {
+    let a = arg(ai)
+    if a == "-o" or a == "--out" {
+      if ai + 1 >= arg_count() { err(`ludic schema: {a} needs a file\n`);  return 2 }
+      ai += 1
+      dest = arg(ai)
+    }
+    else if a == "--unsafe" { g_unsafe_build = true }
+    else if a[0] == '-' {
+      err(`ludic schema: unknown option {a}\n`)
+      err("  usage: ludic schema [file] [-o FILE] [--unsafe]\n")
+      return 2
+    }
+    else { src = a }
+    ai += 1
+  }
+  let entry = find_entry(src)
+  if entry == "" { return no_entry() }
+  ensure_ludicc()
+  var target = dest
+  if target == "" { target = tmp_path("schema.json") }
+  let errs = tmp_path("schema.err")
+  if not shq(`{ludicc()} --headless{unsafe_flag()} {sh_single(entry)} --emit-schema {sh_single(target)} 2> {errs}`) {
+    err(`ludic schema: {entry} did not compile:\n`)
+    err(capture(`grep -i error {errs} | head -20`))
+    return 1
+  }
+  if dest == "" {
+    let s = read_file(target)
+    if s == null { err("ludic schema: the compiler wrote no schema (is it older than ludic schema?)\n");  return 1 }
+    out(s)
+  }
+  return 0
+}