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>

COMPILING.md

@@ -73,6 +73,8 @@ point at a different LLVM toolchain if you have one.
 | a windowed native executable | `ludicc game.ludic -o build/game` |
 | a headless executable | `ludicc game.ludic --headless -o build/game` |
 | the IR, to read | `ludicc src.ludic --emit-llvm -o src.ll` |
+| the schema an editor reads (records, registries and their entries, consts) | `ludicc src.ludic --emit-schema schema.json` |
+| every error, as a JSON array on stdout | `ludicc src.ludic --check --diagnostics=json` |
 | a shared library † | `ludicc lib.ludic --shared -o build/liblib.dylib` |
 | a game that runs in a browser † | `ludicc game.ludic --target wasm32-unknown-unknown -o build/web/game.wasm` |
 | an object file † | `ludicc src.ludic -c -o src.o` |