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/buf.ludic

@@ -4,26 +4,26 @@
 
 property Buf { data: ptr = null, len: int = 0, cap: int = 0 }
 
-fn buf_new() -> Buf {
+function buf_new() -> Buf {
   let b = new Buf
   b.cap = 256
   b.data = bytes(b.cap)
   b.len = 0
   return b
 }
-fn buf_ensure(b: Buf, extra: int) -> void {
+function buf_ensure(b: Buf, extra: int) -> void {
   if b.len + extra + 1 <= b.cap { return }
   while b.len + extra + 1 > b.cap { b.cap = b.cap * 2 }
   b.data = resize(b.data, b.cap)
 }
-fn buf_putc(b: Buf, c: int) -> void {
+function buf_putc(b: Buf, c: int) -> void {
   buf_ensure(b, 1)
   b.data[b.len] = c
   b.len = b.len + 1
 }
-fn buf_puts(b: Buf, s: ptr) -> void {
+function buf_puts(b: Buf, s: ptr) -> void {
   var i = 0
   while s[i] != 0 { buf_putc(b, s[i]); i = i + 1 }
 }
-fn buf_puti(b: Buf, n: int) -> void { buf_puts(b, itoa(n)) }
-fn buf_str(b: Buf) -> ptr { b.data[b.len] = 0; return b.data }
+function buf_puti(b: Buf, n: int) -> void { buf_puts(b, itoa(n)) }
+function buf_str(b: Buf) -> ptr { b.data[b.len] = 0; return b.data }