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/vscode/syntaxes/ludic.tmLanguage.json

@@ -161,7 +161,7 @@
     },
     "keyword": {
       "patterns": [
-        { "name": "keyword.control.ludic", "match": "\\b(if|else|when|while|for|in|match|machine|become|enter|return|spawn|despawn|enable|disable|where|break|continue|new)\\b" },
+        { "name": "keyword.control.ludic", "match": "\\b(if|else|while|for|in|match|machine|become|return|spawn|despawn|enable|disable|where|break|continue|new)\\b" },
         { "name": "keyword.operator.logical.ludic", "match": "\\b(and|or|not)\\b" },
         { "name": "keyword.other.clause.ludic", "match": "\\b(phase|query|reads|writes|on)\\b" },
         { "name": "keyword.other.ludic", "match": "\\b(import|extern)\\b" },