refactor(lang): rename the fn keyword to function

Expand the function-declaration keyword to the full word across the whole
language and toolchain:
  fn name(...) -> T { ... }   ->   function name(...) -> T { ... }

Done as a self-hosting migration: teach the parser both spellings, reseed,
rewrite every .ludic definition to `function`, then drop `fn`. The compiler
now rejects `fn`. Touches the parser, all selfhost/tools/runtime/example/test
sources, the grammars (TextMate shared+vscode, ludic_syntax.h, JetBrains
LudicTokens.kt), the LSP and formatter, the Python doc/vocab tools
(check-impl, check-docs, validate, palette, test-lsp), and the docs
(fences, prose, kw-fn -> kw-function).

Reseeded; C-free bootstrap fixpoint holds. All suites green (45 regression,
24 self-host, 29 tool); the docs site generates and check.py passes.

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

tools/docgen/check-impl.py

@@ -33,12 +33,12 @@ def read(*parts):
 
 
 def fn_body(text, name):
-    """The source of `fn <name>` up to the next top-level `fn ` (or EOF)."""
-    m = re.search(r"^fn %s\b" % re.escape(name), text, re.M)
+    """The source of `function <name>` up to the next top-level `function ` (or EOF)."""
+    m = re.search(r"^function %s\b" % re.escape(name), text, re.M)
     if not m:
         return ""
     rest = text[m.end():]
-    nxt = re.search(r"^fn ", rest, re.M)
+    nxt = re.search(r"^function ", rest, re.M)
     return text[m.start(): m.end() + (nxt.start() if nxt else len(rest))]
 
 

tools/docgen/inventory.json

@@ -266,7 +266,7 @@
     "kw-const",
     "kw-var",
     "kw-let",
-    "kw-fn",
+    "kw-function",
     "kw-return",
     "kw-import",
     "kw-extern",

tools/docgen/palette.py

@@ -269,7 +269,7 @@ def emit_ludic(path):
   lines.append("# Edit the palette there and regenerate; do not hand-edit this file.")
   lines.append("# ============================================================================")
   lines.append("")
-  lines.append("fn color_lookup(name: ptr) -> int {")
+  lines.append("function color_lookup(name: ptr) -> int {")
   for (nm, hexv, grp) in PALETTE:
     lines.append(f'  if (name == "{nm}") {{ return 0x{hexv:06X} }}')
   lines.append("  return -1")

tools/docgen/validate.py

@@ -5,7 +5,7 @@ Full programs (start with program/module) compile directly; fragments are wrappe
 import os, re, subprocess, tempfile, sys
 REPO=os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__)))); LC=os.path.join(REPO,"bin","ludicc")
 FENCE=re.compile(r"```ludic\n(.*?)\n```", re.S)
-DECL=("program","property","model","handler","enum","fn","extern","const","var","ui","event","module","import","scene","machine")
+DECL=("program","property","model","handler","enum","function","extern","const","var","ui","event","module","import","scene","machine")
 
 def first_kw(body):
     for l in body.split("\n"):