Bug Description

Case-insensitive patterns like (?i)HELLO fail to match lowercase text hello when unanchored.

Reproduction

package main

import (
    "fmt"
    "regexp"
    "github.com/coregx/coregex"
)

func main() {
    pattern := `(?i)HELLO`
    input := "hello"

    // stdlib - WORKS
    reStd := regexp.MustCompile(pattern)
    fmt.Println("stdlib:", reStd.FindAllString(input, -1)) // [hello]

    // coregex - FAILS
    reCg := coregex.MustCompile(pattern)
    fmt.Println("coregex:", reCg.FindAllString(input, -1)) // []
}

Root Cause

The literal extractor in literal/extractor.go:129-135 ignores the FoldCase flag from syntax.Regexp. It extracts HELLO as a literal prefix, and the prefilter searches for HELLO case-sensitively, missing hello.

Impact

All unanchored case-insensitive patterns are affected:

• (?i)abc on ABC works (finds first match)
• (?i)abc on ABCabc fails (misses lowercase abc)

Anchored patterns work correctly because they bypass prefilter.

Proposed Fix

Option 1: Don't extract literals when FoldCase flag is set (safe, simple)
Option 2: Expand case-insensitive literals into all variations (complex)