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>

examples/scenes.ludic

@@ -16,7 +16,7 @@ program SceneDemo {
       handler Tick phase Update {
         setreg(R_N, reg(R_N) + 1)
         print_int(100 + reg(R_N))
-        if reg(R_N) >= 2 { enter Play }
+        if reg(R_N) >= 2 { become Play }
       }
     }
   }