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>

examples/lib/combat.ludic

@@ -16,7 +16,7 @@ program Combat {
   const CRIT_MULT: int = 2
 
   # damage after armour, with a crit multiplier applied on an exact roll
-  @export fn damage(attack: int, armour: int, roll: int) -> int {
+  @export function damage(attack: int, armour: int, roll: int) -> int {
     var raw = attack - armour
     if raw < 1 { raw = 1 }
     if roll == 20 { raw = raw * CRIT_MULT }
@@ -25,7 +25,7 @@ program Combat {
 
   # how many hits to drop a target — the loop is here so the caller cannot
   # accidentally disagree with `damage` about rounding
-  @export fn hits_to_kill(hp: int, attack: int, armour: int) -> int {
+  @export function hits_to_kill(hp: int, attack: int, armour: int) -> int {
     var left = hp
     var n = 0
     while left > 0 {
@@ -35,12 +35,12 @@ program Combat {
     return n
   }
 
-  @export fn xp_for(level: int, kills: int) -> int {
+  @export function xp_for(level: int, kills: int) -> int {
     return curve(level) * kills
   }
 
   # private: not exported, so it is not a symbol in the library
-  fn curve(level: int) -> int {
+  function curve(level: int) -> int {
     return 10 + level * level * 3
   }
 }