feat(folding): open every heading fold by default on file open

Heading-block folds were computed but all collapsed on file open,
so users had to `zR` after every `:edit`. Set `foldlevel=99` once at
ftplugin attach so the fold structure is still there (closeable with
`zc`/`zM`) but the buffer renders fully expanded.

`foldlevel` is window-local; we set it on initial attach only — not
in the Lua BufWinEnter re-apply — so closing folds with `zc` in a
window survives navigating away and back to that buffer. Split
windows still get the same value because Vim copies window-local
options to the new window when splitting.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/test-keymaps-vim.vim

@@ -300,6 +300,12 @@ call s:record(
       \ foldlevel(2) > 0 ? 1 : 0,
       \ 'folding.body_inherits_fold',
       \ 'foldlevel(2)=' . foldlevel(2))
+" ftplugin sets `foldlevel=99` so every heading-block fold starts open
+" rather than collapsed on file open.
+call s:record(
+      \ &l:foldlevel == 99 ? 1 : 0,
+      \ 'folding.starts_with_all_folds_open',
+      \ '&l:foldlevel=' . &l:foldlevel)
 
 " ===== Wrap up =====