fix: Bypass Up navigation when Down fails for childless nodes

[zharinov]

Summary

• Pass skip_exit to named node children compilation to bypass Up when Down fails
• Fix star quantifier's first_iterate to use match_exit for internal retry exhaustion
• Fix optional quantifier to emit null injection on retry exhaust path

Why

When a named node has zero children at runtime (e.g., identifier), Down navigation fails immediately. The skip path was still executing Up, ascending one level too high and breaking outer quantifier iteration.

This fix distinguishes between:

• Down fails (childless node): bypass Up via skip_exit
• Retry exhausts (Down succeeded, pattern fails): include Up via match_exit