Phase 7b: null literal + x == null (retire ptr_null/ptr_is_null)

`null` is now a real pointer literal and null-tests are comparisons, instead of
`ptr_null()` and `ptr_is_null(x)`:

  ptr_null()          -> null
  ptr_is_null(x)      -> (x == null)
  not ptr_is_null(x)  -> (x != null)

Mechanics: a new E_NULL primary (`null`, like true/false) lowers to the `null`
pointer; emit_bin's comparison path now picks `ptr` vs `i32` from operand type
(via llty), so `==`/`!=` work on any pointer/record/slice. The two intrinsics are
deleted.

Two reseeds: (A) add the literal + ptr comparison keeping the intrinsics; (B)
migrate all 182 call sites (compiler + runtime, via a balanced-paren script that
skips string-literal args and rewrites `not ptr_is_null` to `!= null`) and delete
the intrinsics. Node/Val/Buf/Tok field defaults now read `ptr = null`.

Vocabulary drops the two from LUDIC_INTRINSICS; `null` joins true/false as a
language constant (grammar + ludic_syntax.h). LANGUAGE.md notes the literal.
Reseeded (21664 lines); C-free fixpoint holds; goldens identical; 17/17; vocab +
doc-fences clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/io.ludic

@@ -3,7 +3,7 @@
 
 fn read_file(path: str) -> ptr {
   let f = file_open(path, "rb")
-  if ptr_is_null(f) { return ptr_null() }
+  if (f == null) { return null }
   file_seek(f, 0, 2)
   let n = file_tell(f)
   file_seek(f, 0, 0)