refactor: merge alternation wrapper effects into body instructions

[zharinov]

Summary

• Extend CaptureEffects with pre field for entry-point effects
• Propagate pre effects to first instruction, post to last
• Tagged alternations: emit Enum as pre_effect, EndEnum as post_effect
• Untagged alternations: emit null injection as pre_effect on branch body

Why

Before: 3 instructions per tagged branch (Enum epsilon → match → EndEnum epsilon)
After: 1 instruction per branch with merged effects

Before: 2 instructions per untagged branch (Null Set epsilon → match)
After: 1 instruction with null injection as pre_effect

For 30-branch tagged alternation: ~99 → 39 instructions (~60% reduction).