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>

selfhost/backend/emit_float.ludic

@@ -135,6 +135,10 @@ function fp_promote_note(from: pointer, t: pointer) -> void {
   if g_warn_promote == 0 { return }
   var file = g_err_file
   if (file == null) { file = "" }
+  if g_diag_json {
+    diag_add(file, g_err_line, 0, "warning", `a computed {from} is promoted to {t}`)
+    return
+  }
   let m = `{file}:{itoa(g_err_line)}: warning: a computed {from} is promoted to {t}\n`
   file_write(file_stderr(), m, len(m))
 }