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>

selfhost/io.ludic

@@ -1,7 +1,7 @@
 # io.ludic — reading the input file and writing the output, plus tiny stdio.
 # The compiler reads one .ludic file whole and emits LLVM IR text to stdout.
 
-fn read_file(path: str) -> ptr {
+function read_file(path: str) -> ptr {
   let f = file_open(path, "rb")
   if (f == null) { return null }
   file_seek(f, 0, 2)
@@ -15,7 +15,7 @@ fn read_file(path: str) -> ptr {
 }
 
 # length of a NUL-terminated buffer
-fn cstr_len(s: ptr) -> int {
+function cstr_len(s: ptr) -> int {
   var n = 0
   while s[n] != 0 { n = n + 1 }
   return n