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>

tools/editors/jetbrains/src/main/kotlin/io/ludic/ide/LudicTokens.kt

@@ -68,8 +68,8 @@ object LudicVocabulary {
         // compiler intrinsics: the floor the Ludic-written runtime stands on
         "mem_alloc", "mem_realloc", "mem_free", "mem_copy", "mem_set", "peek8", "poke8", "peek32",
         "os_argc", "os_arg", "file_stderr",
-        "poke32", "peekp", "pokep", "peekf", "pokef", "ptr_add", "ptr_null",
-        "ptr_is_null", "as_fixed", "as_int", "file_open", "file_read", "file_write",
+        "poke32", "peekp", "pokep", "peekf", "pokef", "ptr_add",
+        "as_fixed", "as_int", "file_open", "file_read", "file_write",
         "file_seek", "file_tell", "file_close", "read_byte", "write_byte", "print_str",
         "str_len", "os_exit", "os_time",
         "is_windowed", "game_title", "win_open", "win_poll", "win_present",