feat: add artist_initials, artists, explicit, release template variables

[np3ir]

Summary

Adds four new template variables and one config option to improve filename/folder customization:

• {artist_initials} — first letter of album_artist (ASCII-normalized, # for non-alpha). Useful for grouping artists into letter buckets (A/, B/, ...).
• {artists} — artist string with configurable separator. Splits the combined artistName on " & " and ", " and rejoins with artist_separator. The original {artist} is unchanged for backward compatibility.
• {explicit} — "(explicit)" when the track is rated explicit, empty string otherwise.
• {release} — album release type: "ALBUM", "SINGLE", or "COMPILATION". Derived from the compilation flag and trackCount.
• artist_separator (config/CLI) — separator used for {artists}. Default: " & " (backward compatible).
• music_video_output_path (config/CLI) — separate output directory for music videos. When set, music videos go here instead of output_path.
• use_fullwidth_replacements (config/CLI, default true) — replace forbidden Windows filename characters with Unicode full-width equivalents (：, ＊, ？, etc.) instead of underscores, producing cleaner filenames where punctuation is visually preserved.

Example config

output_path = Z:/Music
music_video_output_path = G:/Videos
artist_separator = " / "
use_fullwidth_replacements = true
album_folder_template = {artist_initials}/{album_artist}/({date:%Y}) {album} ({release})
single_disc_file_template = {track:02d}. {artists} - {title}{explicit}

produces:

Z:/Music/A/Alex Cuba/(2020) Solo Mía (SINGLE)/01. Alex Cuba / Leonel García - Amor Secreto (explicit).m4a
G:/Videos/A/Alex Cuba/(2020) Solo Mía (SINGLE)/01. Alex Cuba / Leonel García - Solo Mía (explicit).mp4

Changes

• gamdl/interface/types.py — MediaTags.release_type: str = None
• gamdl/interface/song.py — populate release_type in get_tags()
• gamdl/interface/music_video.py — populate release_type in get_tags()
• gamdl/downloader/constants.py — FULLWIDTH_REPLACEMENTS map
• gamdl/downloader/base.py — all new params, helper methods, formatter vars

Test plan

• Download a single-artist album — {artist_initials} = correct letter, {release} = ALBUM
• Download a single-track release — {release} = SINGLE
• Download a compilation — {release} = COMPILATION
• Download a multi-artist track — {artists} splits and rejoins with configured separator
• Explicit track — {explicit} = "(explicit)", clean track — {explicit} = ""
• Album name with : — produces ： not _
• Music video URL with music_video_output_path set — goes to separate directory
• Default artist_separator = " & " — existing templates unchanged