fix(restore): preserve live buffer and cursor across close/restore

[WilliamHsieh]

popup.opts.target_bufnr / lnum / col are captured once by the adapter at
peek time and never refreshed. If the user navigates inside a popup (gd
/ gf / :edit / direct cursor moves), the popup's window state diverges
from opts: the window shows a different buffer or has the cursor at a
different position. close_all + restore then reopens what was originally
peeked, not what the user was last viewing -- losing their navigation.

Snapshot the popup's live state into opts at two points:

1. Popup:snapshot_state() reads nvim_win_get_buf + nvim_win_get_cursor
   and writes back to self.opts. Idempotent; rerunning is a no-op if
   the popup hasn't moved since the last snapshot.

2. Two callers cover the close paths:

   • Popup:close (the programmatic close_all path; close_all suppresses
     WinLeave via eventignore so the autocmd below cannot catch it here).
   • A new WinLeave autocmd in init.lua (the focus-leaves and external
     :close paths; Popup:close is not invoked when the popup closes via
     nvim's own WinClosed -> on_popup_closed dispatch).

Every close path is covered by at least one snapshot point.

The autocmd reads is_overlook_popup via nvim_win_get_var rather than
vim.w because tests reassign vim.w = {} for isolation, which detaches
the proxy from real window-locals.

opts.col is 1-indexed (the adapters use vim.fn.getpos);
nvim_win_get_cursor returns 0-indexed col, so snapshot_state adds 1 for
the round-trip with set_cursor_position's math.max(0, col - 1) on
restore.

Adds two regression tests: one for the WinLeave path (user switches
focus before close_all), one for the Popup:close path (user is still
inside the popup at close_all time).

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com