fix: preserve manually opened folds after text changes

[tasso-cs]

Summary

When using zM to close all folds (foldlevel=0) and then zo to manually open a fold, editing text would cause the fold to close again.

Root cause

1. syncFoldedLines() wasn't called before getRangesFromExtmarks(), so ufo didn't know about manually opened folds
2. When folds are recreated with foldlevel=0, all folds close, overriding manually opened ones

Fix

• Call fb:syncFoldedLines(winid) before getRangesFromExtmarks() to sync ufo's internal state with Vim's actual fold state
• After restoring foldlevel, if it's 0, explicitly reopen folds that should stay open (those not in rowPairs)

Steps to reproduce (before fix)

1. Open a file with folds (e.g., markdown with headers)
2. Run zM to close all folds
3. Run zo to open one fold
4. Edit text (e.g., x to delete a character)
5. Bug: The fold closes again

Test plan

• Tested manually: after fix, manually opened folds stay open after text edits
• Verified both FFI and non-FFI code paths are updated

[tasso-cs]

@kevinhwang91 let me know if this is of interest to review / merge. thank you :)

[c-tesch]

This happens when fully fold is the default too:

vim.opt.foldlevelstart = 0

whatever changes I make, x, d, or leave insert mode folds all sections