Phase 6a: remove the redundant when and enter keywords

Cohesion audit found two keywords that are pure duplicates:
  - `when c { }` produced a byte-identical S_IF to an else-less `if c { }`
    (no distinguishing flag) — so it was a second spelling of the same node.
  - `enter Name` produced identical codegen to `become Name`: it set an
    `ival` flag that emit_become never reads.

Both removed from the parser. `if` already parses with an optional `else`, so
nothing is lost. examples/scenes.ludic (an uncompiled design sketch) and the
LANGUAGE.md scenes section now use `become Name` for scene transitions.

Vocabulary synced (ludic_syntax.h, TextMate grammar, LudicTokens.kt);
check-vocabulary clean. Reseeded (22148 lines); C-free fixpoint holds;
goldens identical; 17/17.

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

tools/ludic-tools/ludic_syntax.h

@@ -66,8 +66,8 @@ static const char* LUDIC_KW_RESERVED[] = {
     "scene","layer","start", 0
 };
 static const char* LUDIC_KW_STMT[] = {
-    "let","return","if","else","when","while","for","in","spawn","despawn",
-    "enable","disable","match","machine","state","become","enter","where",
+    "let","return","if","else","while","for","in","spawn","despawn",
+    "enable","disable","match","machine","state","become","where",
     "and","or","not","break","continue","new", 0
 };
 static const char* LUDIC_TYPES[] = {