docs: correct stale references across LANGUAGE, README, COMPILING, CONTRIBUTING

- LANGUAGE.md: real diagnostic format, list literals, Font.load / Ui.*
  (no `reg`/`set_reg`, no `/Handler/Library` typo), `extern function`,
  existing example links, Input.key(); builtins table trimmed to what exists
- COMPILING.md: pipeline names selfhost/ (not compiler/*.c), `program`
  instead of game/module, all thirteen win_* entry points by group
- README.md: the window seam is not "five" functions
- CONTRIBUTING.md: docs are checked with `x docs-gen` / `x docs-check`
- docs/language: kw-ui and fn-ui_build use the namespaced API;
  @ClearColor documents constant expressions; examples/README lists
  operators.ludic

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -61,7 +61,7 @@ The stdlib lives in the runtime (`runtime/`) and is surfaced as namespaces
    and register its id in `tools/docgen/inventory.json`. Each documented
    namespace gets exactly **one** directory (the docs check enforces this).
 3. Add or extend an example under `examples/` and a case in the test suite.
-4. Run `python3 tools/docgen/gen.py && python3 tools/docgen/check.py` — the
+4. Run `bin/x docs-gen --out build/pages && bin/x docs-check build/pages` — the
    check fails if any inventory symbol lacks a page or is still seed text.
 5. Add a **changeset** for the user-facing change: a small file under
    [`changes/`](changes/README.md) with a `bump:` level and a one-line summary.