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

@@ -48,8 +48,8 @@ object LudicVocabulary {
         "phase", "query", "reads", "writes", "on"
     )
     val STMT = setOf(
-        "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"
     )
     val PRIMITIVES = setOf("int", "fixed", "bool", "entity", "str", "ptr", "void")