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/tests/control.ludic

@@ -1,5 +1,5 @@
 program T {
-  fn fib(n: int) -> int { if n < 2 { return n };  return fib(n-1) + fib(n-2) }
+  function fib(n: int) -> int { if n < 2 { return n };  return fib(n-1) + fib(n-2) }
   entry {
     print(fib(10))                 # 55
     var s = 0

selfhost/tests/long.ludic

@@ -1,5 +1,5 @@
 program T {
-  fn wide(a: long, b: int) -> long { return a * b }   # int arg widens to i64
+  function wide(a: long, b: int) -> long { return a * b }   # int arg widens to i64
 
   entry {
     let big: long = 1000000       # int literal widens to a long

selfhost/tests/match_bits.ludic

@@ -1,5 +1,5 @@
 program T {
-  fn classify(c: int) -> int {
+  function classify(c: int) -> int {
     match c { 65, 66 => { return 1 };  67 => { return 2 };  _ => { return 9 } }
     return 0
   }

selfhost/tests/structs.ludic

@@ -1,6 +1,6 @@
 program T {
   property P { x: int = 0, y: int = 7, next: P }
-  fn bump(p: P) -> void { p.x = p.x + 100 }
+  function bump(p: P) -> void { p.x = p.x + 100 }
   entry {
     let a = new P
     print(a.y)            # 7 default