feat(tooling): port the docgen site generator to Ludic (no Python) (#41)

Follow-up to #31: the doc/lint/grammar checks moved to Ludic there; this ports
the remaining docgen piece (gen.py / check.py / palette.py) so nothing in the
documentation pipeline is Python any more.

Three new `x` subcommands, all in Ludic and compiled by Ludic:

  - x docs-gen [--out DIR]  the static-site generator: parses docs/language/**
    front-matter + bodies (fences, Parameters:), builds the section/symbol
    model, reads the asset templates, and emits every page + ns/color/api pages
    + the landing page + ludic-highlight.js + symbols.json + .nojekyll.
  - x docs-check [DIR]      the coverage / integrity guard (required files, a
    page per inventory.json symbol, duplicate-token and one-dir-per-namespace
    guards, highlighter link targets).
  - x docs-palette          the named-colour source of truth: the palette table
    moved into tools/x/docgen.ludic, emitting emit_color.ludic (pointer, not
    ptr) + palette.json.

Verified against the Python oracle: `x docs-gen` reproduces all 466 output files
BYTE-FOR-BYTE (a Ludic json.dumps/html.escape/front-matter port — ordered dicts,
indent=2 vs compact, ensure_ascii \uXXXX, codepoint-aware truncation), and
`x docs-check` matches check.py's pass/fail output. Wired into `x test` as a
gate (docs-gen -> docs-check on a fresh site; docs-palette stays byte-identical).

CI swap: ci.yml and docs.yml call the Ludic generator; docs.yml drops the
python:3.12 container and bootstraps the toolchain from the IR seed instead.
tools/docgen/{gen,check,palette}.py deleted; only assets/ + inventory.json
remain. Completes #31's criterion 3.

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

selfhost/backend/stdlib/emit_color.ludic

@@ -5,8 +5,8 @@
 # no allocation, identical codegen to writing the hex by hand. Unknown names are
 # a compile error (color_lookup returns -1, which emit_expr reports).
 #
-# GENERATED by scratchpad/palette.py from the single source-of-truth palette.
-# Edit the palette there and regenerate; do not hand-edit this file.
+# GENERATED by `x docs-palette` from the single source-of-truth palette table
+# in tools/x/docgen.ludic. Edit the palette there and regenerate; do not hand-edit.
 # ============================================================================
 
 function color_lookup(name: pointer) -> int {