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/main.ludic

@@ -15,7 +15,7 @@
 # reseed.sh rely on, so the bootstrap is untouched.
 
 # basename: the part of a path after the last '/'.
-fn base_name(path: ptr) -> ptr {
+function base_name(path: ptr) -> ptr {
   var last = 0 - 1
   var i = 0
   while path[i] != 0 { if path[i] == 47 { last = i };  i = i + 1 }
@@ -23,7 +23,7 @@ fn base_name(path: ptr) -> ptr {
 }
 
 # drop a trailing ".ludic" if present
-fn strip_ludic(name: ptr) -> ptr {
+function strip_ludic(name: ptr) -> ptr {
   let n = len(name)
   if n > 6 {
     if (name[n - 6..n] == ".ludic") { return name[0..n - 6] }
@@ -32,21 +32,21 @@ fn strip_ludic(name: ptr) -> ptr {
 }
 
 # env var with a fallback when unset
-fn getenv_or(name: ptr, dflt: ptr) -> ptr {
+function getenv_or(name: ptr, dflt: ptr) -> ptr {
   let v = getenv(name)
   if (v == null) { return dflt }
   return v
 }
 
 # guarantee a directory string ends in '/' so path_join concatenates cleanly
-fn ensure_slash(d: ptr) -> ptr {
+function ensure_slash(d: ptr) -> ptr {
   let n = len(d)
   if n == 0 { return d }
   if d[n - 1] == 47 { return d }
   return (d + ("/"))
 }
 
-fn die(msg: ptr) -> void {
+function die(msg: ptr) -> void {
   file_write(file_stderr(), msg, len(msg))
   exit(1)
 }