refactor(selfhost): reorganise into concern-based subdirectories

Split the flat 38-file selfhost/ into concern-based subdirectories:

  frontend/        lex, parse, parse_game, ast
  support/         str, buf, io
  backend/         core IR + expression/statement lowering
  backend/game/    ECS/scene/event/world lowering
  backend/stdlib/  the namespaced Math.*/Text.*/Crypto.*/… intrinsics

and split the three oversized emitters at responsibility boundaries so
no file mixes concerns:

  emit_game.ludic  -> + emit_world.ludic         (reflection world table,
                                                  tick helpers, @main synthesis)
  emit_expr.ludic  -> + emit_call.ludic          (namespaced builtins, call
                                                  lowering, expr dispatch)
  emit_text.ludic  -> + emit_text_prelude.ludic  (emitted string-builder runtime)

FRAGS in tools/x/selfhost.ludic is updated to the new paths with the link
order preserved, and the Python doc/vocabulary tooling is updated to walk
the new layout. Because the build is a plain in-order concatenation and
every split lands on a blank-line boundary, the regenerated seed is
byte-identical: `x reseed` leaves selfhost/ludicc.seed.ll unchanged,
`x bootstrap-cfree` still reaches its fixed point, and both `x test` (56)
and `x selfhost-test` (29, incl. golden renders) stay green.

Closes #29

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

tools/docgen/check-impl.py

@@ -7,7 +7,7 @@ never written. So the set of things that MUST have a doc page is read straight
 from the implementation, and every one is required to have a page (and every
 namespace-method page is required to correspond to something real):
 
-  * namespace methods   selfhost/emit_expr.ludic `emit_ns_call` + the
+  * namespace methods   selfhost/backend/emit_call.ludic `emit_ns_call` + the
                         `is_<ns>_ns` predicates it delegates to (Math/Text/List)
   * keywords            tools/ludic-tools/ludic_syntax.h  LUDIC_KW_* (minus
                         LUDIC_KW_RESERVED, which is not-yet-implemented)
@@ -45,20 +45,23 @@ def fn_body(text, name):
 def all_selfhost_source():
     src = ""
     d = os.path.join(ROOT, "selfhost")
-    for fn in sorted(os.listdir(d)):
-        if fn.endswith(".ludic"):
-            src += read("selfhost", fn) + "\n"
+    files = []
+    for dirpath, _dirs, names in os.walk(d):
+        for fn in names:
+            if fn.endswith(".ludic"):
+                files.append(os.path.join(dirpath, fn))
+    for path in sorted(files):
+        src += open(path, encoding="utf-8").read() + "\n"
     return src
 
 
 def compiler_ns_methods():
     """{Namespace: set(method)} the compiler actually dispatches on."""
-    expr = read("selfhost", "emit_expr.ludic")
-    body = fn_body(expr, "emit_ns_call")
-    if not body:
-        problems.append("emit_ns_call not found in selfhost/emit_expr.ludic")
-        return {}
     allsrc = all_selfhost_source()
+    body = fn_body(allsrc, "emit_ns_call")
+    if not body:
+        problems.append("emit_ns_call not found in selfhost/ sources")
+        return {}
     result = {}
     # each `if (ns == "X")` opens a block that runs to the next such marker
     chunks = re.split(r'if \(ns == "', body)

tools/docgen/palette.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 # Single source of truth for Ludic's named color palette.
-# Emits: selfhost/emit_color.ludic (compiler lookup) and palette.json (docs).
+# Emits: selfhost/backend/stdlib/emit_color.ludic (compiler lookup) and palette.json (docs).
 import json, sys, os
 
 # (Name, 0xRRGGBB, group). Names are PascalCase, unique. >200 entries.
@@ -293,6 +293,6 @@ if __name__ == "__main__":
   names = guard_check()
   root = os.path.dirname(os.path.abspath(__file__))
   repo = "/Users/orkuncakilkaya/workspace/gpp"
-  emit_ludic(os.path.join(repo, "selfhost", "emit_color.ludic"))
+  emit_ludic(os.path.join(repo, "selfhost", "backend", "stdlib", "emit_color.ludic"))
   emit_json(os.path.join(root, "palette.json"))
   print(f"OK {len(PALETTE)} colors ({len(set(names))} unique names)")