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>

COMPILING.md

@@ -103,8 +103,8 @@ A source file opens with `game Name { … }` or `module Name { … }`.
 ```ludic
 # doc-check: skip — illustrative: elided body
 program Combat {
-  @export fn damage(attack: int, armour: int, roll: int) -> int { … }
-  fn curve(level: int) -> int { … }          # private: not a symbol
+  @export function damage(attack: int, armour: int, roll: int) -> int { … }
+  function curve(level: int) -> int { … }          # private: not a symbol
 }
 ```
 
@@ -118,7 +118,7 @@ Those are ordinary C-ABI symbols, so anything that can call a shared library can
 call Ludic. To call them from another Ludic program, declare them and link:
 
 ```ludic
-extern fn damage(attack: int, armour: int, roll: int) -> int = "damage"
+extern function damage(attack: int, armour: int, roll: int) -> int = "damage"
 ```
 
 ```bash