fix: respect Blacklist/Whitelist options for single-language scripts

[Yanhu007]

Fixes #28

Problem

Languages detected by unique script (Han→Mandarin, Bengali→Bengali, Hangul→Korean, etc.) bypassed Options entirely. Blacklist and Whitelist were only checked for multi-language scripts (Latin, Cyrillic, etc.) that go through detectLangInProfiles.

opts := Options{Blacklist: map[Lang]bool{Cmn: true}}
info := DetectWithOptions("你好世界", opts)
// Expected: lang=unknown (blacklisted)
// Got:      lang=Mandarin, confidence=1.0 (blacklist ignored!)

Fix

Add filterLangByOptions helper that checks Blacklist/Whitelist before returning single-language script detections. If the language is filtered out, returns (-1, 0) (unknown).

Testing

All existing tests pass. Manually verified:

• Chinese text with Cmn blacklisted → returns unknown ✅
• Chinese text with Japanese-only whitelist → returns unknown ✅
• Chinese text without options → returns Mandarin ✅