fix(grep): align preview highlight stripping with Rust query parser

The Lua heuristic in highlight_grep_matches used a simple prefix check
(^[*!/] or ^.) to strip constraints. This diverged from the Rust
GrepConfig parser in several ways:

- Multi-word queries like 'foo bar *.rs' only highlighted 'foo'
- Constraint prefixes like type:rust were not stripped
- Tokens starting with '.' were incorrectly treated as constraints
- Escaped constraint tokens (e.g. \*.config) were not handled

Replace the heuristic with _is_grep_constraint() that matches the
Rust parser's actual GrepConfig rules: extensions (*.rs), path segments
(/src/), exclusions (!test), type filters (type:rust), and path-oriented
globs. Use all text parts joined with space for highlighting, matching
grep_text() on the Rust side.

Fixes #331

Author: mvanhorn

lua/fff/location_utils.lua

@@ -134,6 +134,45 @@ function M.highlight_location(bufnr, location, namespace)
   return #extmarks > 0 and extmarks or nil
 end
 
+--- Check if a token is a grep constraint (matches Rust GrepConfig parser rules).
+--- Keeps highlight stripping aligned with the actual query parser so preview
+--- highlights match the grep results.
+--- @param token string A single whitespace-delimited query token
+--- @return boolean
+function M._is_grep_constraint(token)
+  if token == '' then return false end
+
+  -- Escaped tokens (leading \) are never constraints in the Rust parser
+  if token:sub(1, 1) == '\\' and #token > 1 then return false end
+
+  local first = token:sub(1, 1)
+
+  -- Extension: *.rs, *.{ts,tsx} (but not bare "*" or "*.")
+  if first == '*' then
+    if token == '*' or token == '*.' then return false end
+    if token:match('^%*%.') then return true end
+    return false
+  end
+
+  -- Exclusion: !foo, !*.rs
+  if first == '!' and #token > 1 then return true end
+
+  -- Path segment with leading slash: /src/, /lib
+  if first == '/' then return true end
+
+  -- Path segment with trailing slash: src/, lib/
+  if token:sub(-1) == '/' then return true end
+
+  -- Glob: tokens containing / or brace expansion with letters like {ts,tsx}
+  if token:find('/') then return true end
+  if token:match('{%a.*,%a.*}') then return true end
+
+  -- Type filter: type:rust
+  if token:match('^type:') then return true end
+
+  return false
+end
+
 --- Highlight all occurrences of a grep pattern in the preview buffer.
 --- For plain text and regex modes: highlights every match on all loaded lines
 --- using Lua string.find with the query text.
@@ -178,16 +217,22 @@ function M.highlight_grep_matches(bufnr, location, namespace)
 
   local query = location.grep_query
 
-  -- Extract the actual search text from the grep query (strip file constraints like *.rs /src/)
-  -- The query parser uses space-separated tokens; the first non-constraint token is the pattern.
-  -- Simple heuristic: strip tokens that look like constraints (start with *, /, or !)
-  local search_text = query
+  -- Extract search text by stripping constraint tokens, matching the Rust
+  -- query parser's GrepConfig rules (extensions, path segments, globs,
+  -- exclusions, type filters). Escaped leading backslash means "not a
+  -- constraint" in the Rust parser, so we keep those tokens too.
   local parts = vim.split(query, '%s+')
   local text_parts = {}
   for _, part in ipairs(parts) do
-    if part ~= '' and not part:match('^[%*!/]') and not part:match('^%.') then table.insert(text_parts, part) end
+    if part ~= '' and not M._is_grep_constraint(part) then
+      -- Strip leading backslash when it escapes a constraint trigger (*, /, !)
+      -- so that e.g. \*.config highlights "*.config" not "\*.config"
+      if #part > 1 and part:sub(1, 1) == '\\' and part:sub(2, 2):match('[%*!/]') then part = part:sub(2) end
+      table.insert(text_parts, part)
+    end
   end
-  if #text_parts > 0 then search_text = text_parts[1] end
+  local search_text = table.concat(text_parts, ' ')
+  if search_text == '' then search_text = query end
 
   if not search_text or search_text == '' then return nil end