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/ludic-tools/ludic_syntax.h

@@ -53,7 +53,7 @@ typedef struct {
  * declaration keyword, anything stmt() dispatches on is a statement keyword. */
 static const char* LUDIC_KW_DECL[] = {
     "program","import","property","model","enum","ui",
-    "const","var","fn","extern","handler","entry","event","scene", 0
+    "const","var","function","extern","handler","entry","event","scene", 0
 };
 static const char* LUDIC_KW_CLAUSE[] = {
     "phase","query","on","cancellable","public","layer","start", 0