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/net_rt.ludic

@@ -14,7 +14,7 @@
 # a real socket when a program binds `extern fn net_send/net_poll`).
 
 # The authority ships one entity's authoritative synced state to peers.
-fn rt_replicate(e: int) -> void {
+function rt_replicate(e: int) -> void {
   let w = words(512)
   w[0] = e                                 # entity id in the first word
   let n = serialize(e, offset(w, 4))       # synced fields after it
@@ -23,7 +23,7 @@ fn rt_replicate(e: int) -> void {
 
 # A peer drains every inbound snapshot and applies it to the named entity. One
 # datagram per poll (the transport is datagram-preserving), so loop until empty.
-fn rt_receive() -> void {
+function rt_receive() -> void {
   let w = words(512)
   var n = net_poll(w, 2048)
   while n > 0 {