feat: add --exact-lang flag for precise language matching

New --exact-lang CLI flag that enables exact language code matching instead of fuzzy matching. This allows users to get specific regional variants without matching all related variants.

Examples:
- `-l es-419` normally matches all Spanish (es-ES, es-419, es-MX)
- `-l es-419 --exact-lang` matches ONLY es-419 (Latin American Spanish)

Fixes language detection issue where specific variants like es-419 (Latin American Spanish) would match all Spanish variants instead of just close regional variants.

unshackle/core/constants.py

@@ -6,6 +6,7 @@ DOWNLOAD_LICENCE_ONLY = Event()
 
 DRM_SORT_MAP = ["ClearKey", "Widevine"]
 LANGUAGE_MAX_DISTANCE = 5  # this is max to be considered "same", e.g., en, en-US, en-AU
+LANGUAGE_EXACT_DISTANCE = 0  # exact match only, no variants
 VIDEO_CODEC_MAP = {"AVC": "H.264", "HEVC": "H.265"}
 DYNAMIC_RANGE_MAP = {"HDR10": "HDR", "HDR10+": "HDR10P", "Dolby Vision": "DV", "HDR10 / HDR10+": "HDR10P", "HDR10 / HDR10": "HDR"}
 AUDIO_CODEC_MAP = {"E-AC-3": "DDP", "AC-3": "DD"}