fix: live bindings for mutable exports

[Mathicha]

Started as a simple fix so advanced mode could handle Svelte, which exports mutable bindings that get reassigned later (ref) (minimal playground repro).
It snowballed into a pile of fixtures for random export-related edge cases.

co-dev with opus 5 in low effort

[codspeed-hq]

Merging this PR will not alter performance

✅ 3 untouched benchmarks

---

_{Comparing Mathicha:exports (0010cac) with main (3ac0fab)}