fix: Cascade overflow pre-effects to prevent bytecode encoding panic

[zharinov]

Summary

• Fix panic when unlabeled alternations have 5+ branches with unique captures
• Add emit_match_with_cascade() to split overflow pre-effects into epsilon chain
• Add assertions for bytecode field limits (pre/post/neg counts, Up level)

Why

Unlabeled alternations inject Null + Set effects for missing captures per branch. With 5+ branches, this exceeded the 3-bit limit (max 7) for pre_count, causing u16 overflow that corrupted successor addresses and panicked with "range start index 90816 out of range".

The fix cascades overflow effects into leading epsilon transitions, matching the existing pattern for successor overflow.