fix(preview): drop vim.pesc on grep highlight needle (#630)

[gustav-fff]

Closes #630

Root cause

lua/fff/location_utils.lua:207 runs vim.pesc(search_text) on the grep needle before scanning preview lines, but the scanner at lua/fff/location_utils.lua:224 calls string.find(search_line, search_pat, start_pos, true) with plain=true. With plain=true the needle is treated as a literal byte sequence, so the escape turns . into %. and the scanner then looks for the two literal bytes % . in content — which never appears in normal source code. Same failure mode for every other Lua-pattern metachar (-, (, ), +, *, ?, [, ], ^, $, %). Single-word queries without metachars look fine, multi-word queries that happen to contain a . (e.g. file extensions, version strings, member access) lose preview highlighting entirely.

Fix

Drop vim.pesc. Pass the raw needle into string.find(..., true) directly. Smart-case lowering is preserved.

Steps to reproduce

git checkout origin/main
nvim ~/some/repo

Inside Neovim:

1. :lua require('fff').find_files() then toggle to grep mode (default <C-g>), submode plain or regex.
2. Type a query containing a ., e.g. foo.bar or 1.0 or vim.api.
3. Move cursor to a result whose preview line contains the literal string.

Expected: every occurrence of foo.bar in the preview window is highlighted with IncSearch.
Actual: no preview highlights at all (the list row itself still shows the match because that path goes through Rust's match_byte_offsets, not this Lua scanner).

A single-token query without metachars (e.g. foo) highlights correctly, confirming the path works and only the escaping is wrong.

How verified

• Read the scan loop: string.find(search_line, search_pat, start_pos, true) — the 4th argument is plain, which per Lua docs causes the pattern to be treated literally. Combined with vim.pesc, every metachar yields a needle that cannot match real content.
• Manual repro in nvim with query vim.api on this repo, pre-fix: no preview highlights. Post-fix: all vim.api occurrences in the visible preview window highlighted.

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