Three bugs fixed to safely enable promotion for control flow:
1. compute_stack_needs now recurses into IF/DoLoop/Begin bodies,
correctly calculating preload counts for promoted words with
nested control flow (was flat, causing stack underflow).
2. BeginDoubleWhileRepeat rejected from promotion (boot.fth's
-TRAILING uses this pattern, handler had structural bugs).
3. IF/ELSE branches must have same net stack effect for promotion
(BITSSET? has asymmetric branches: 2 items vs 1).
Performance with promotion enabled:
- Factorial: 0.50x (2x faster than gforth)
- Collatz: 0.38x (2.6x faster than gforth)
- All 427 unit tests, 10/11 compliance, 35/35 behavioral pass
ok
4cc71666d5