fix(compile): use exact navigation for alternation branches and definition bodies

[zharinov]

Summary

Fixes the "search scope family of bugs" where the search loop operates below the level of alternation/backtracking semantics, causing cursor advancement to "consume" positions before other branches can try them.

• Alternation branches now use exact navigation (Nav::to_exact()) - branches match at current position only
• Definition bodies now use Nav::StayExact - named definitions match at cursor position, caller owns search

This ensures that when [(A) (B)] is matched where A = (function_call) and B = (juxt_function_call):

1. Branch A tries at current position with exact nav
2. If A fails, backtrack to try B at same position
3. Only after all branches fail does the parent context advance

Previously, A's search loop could advance past positions that B should have tried.

Test plan

• All 751 tests pass
• Named definitions in alternation work: [(A) (B)] returns 2 matches
• Full Gradle pattern works with tagged union output
• Previous JavaScript alternation fix still works