fix(picker_ui): handle 'winfixbuf' when opening selected file

[beixiyo]

Problem

Selecting a file in the picker raises E1513 whenever the source window has winfixbuf enabled:

Vim(edit):E1513: Cannot switch buffer. 'winfixbuf' is enabled
  at lua/fff/picker_ui.lua:2478 (in M.select)

M.select's edit branch checks the current buffer's buftype / modifiable, but not the window-local winfixbuf option. :h winfixbuf is the documented mechanism (Neovim 0.10+) for pinning a window to its buffer; any code path that runs :edit must respect it, otherwise the picker becomes unusable from a pinned window.

Why this isn't an obscure edge case

winfixbuf is the official "pin this window" API, not a niche flag. It fires in realistic flows:

• A user explicitly pins the current file (:set winfixbuf) to keep it as a reference while jumping around — and then opens the picker to look at something else.
• Sessions / view files (:mksession, auto-session, etc.) restore winfixbuf from a previous session.
• Plugins set winfixbuf on certain windows; if the picker's find_suitable_window happens to land on one of those windows, the same error fires even when the source window was a non-file buffer.

Other pickers (e.g. Telescope, Snacks.picker) already handle winfixbuf for this reason.

Reproduction (minimal)

1. :edit any_file.lua
2. :set winfixbuf
3. Open the picker and select another file.

Expected: file opens. Actual: E1513, file is not opened.

Fix

• Add a small helper window_has_winfixbuf(win) (pcall-guarded for Neovim < 0.10 where the option doesn't exist).
• find_suitable_window: skip windows where winfixbuf is set, so we never redirect into a pinned window.
• M.select's edit branch: treat winfixbuf on the current window like a non-modifiable buftype — try find_suitable_window first; if no suitable window exists, fall back to :split so the file still opens without clobbering the pinned window.

Default path (no winfixbuf anywhere) is unchanged: :edit replaces the current buffer as before.

Test scenarios verified

• No winfixbuf → edit replaces current buffer (regression check, default path unchanged)
• Single window with winfixbuf → falls back to :split, original window keeps its pinned buffer
• Multiple windows, current is winfixbuf + another normal window exists → switches to the normal window and :edit
• Quickfix open + main window winfixbuf → split fallback, no error
• :set nowinfixbuf after fix → behavior identical to default path

Notes

• No new dependencies, no behavior change for users not using winfixbuf.
• pcall around nvim_get_option_value('winfixbuf', ...) keeps the patch safe on Neovim versions predating the option.