feat(tooling): port the doc/lint/grammar checks to Ludic (no Python)

Replace the Python doc/lint/vocabulary guards with Ludic equivalents that
run through the `x` task runner, so the checks need no Python interpreter:

  x check-docs         every ```ludic doc fence parses (or is marked)
  x check-impl         every implemented feature has a docs/language page
  x check-vocabulary   vocabulary in sync across grammar / lexer / header / parser
  x lint-asset <file>  validate one editor .json / .xml asset

New fragments: tools/x/json.ludic (a small JSON reader — objects/arrays/
strings with \uXXXX + surrogates/numbers/literals, used by the vocabulary
check's grammar navigation and the asset validator) and tools/x/checks.ludic
(the checks + string helpers + a minimal XML well-formedness validator).

`x test-tools` now runs the vocabulary + docs-coverage checks and the
JSON/XML asset validation through Ludic instead of python3; ci.yml's
docs-coverage step calls `x check-impl` / `x check-vocabulary`. Each port was
verified against its former Python script for exact verdict parity on the
clean tree and on injected drift (a removed keyword, a broken grammar
alternation, an undocumented method).

Deletes the superseded scripts: tools/check-vocabulary.py, tools/check-docs.py,
tools/docgen/check-impl.py, tools/docgen/validate.py. The docgen site
generator (gen.py/check.py/palette.py) and the LSP protocol driver
(test-lsp.py) remain and are tracked separately.

Toolchain unchanged (seed byte-identical); `x test` (56) and `x test-tools`
(29) stay green.

Part of #31

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -75,6 +75,10 @@ jobs:
       - name: Docs cover the implementation
         run: |
           set -eu
+          # check-impl / check-vocabulary are written in Ludic and run through
+          # x — no Python. (check-vocabulary also runs in `x test-tools`.) The
+          # site generator (gen.py) is still Python; its port is tracked.
+          bin/x check-impl
+          bin/x check-vocabulary
           python3 tools/docgen/gen.py --out build/pages
           python3 tools/docgen/check.py build/pages
-          python3 tools/docgen/check-impl.py