Adds Track.drm_preference so a service can pin a track to Widevine or
PlayReady when a manifest advertises both but only one will license it.
Honoured in DASH/HLS selection, the dl licence path and the remote/API
track round-trip. Also fixes quality-keyed CDM maps sending audio to the
default entry, and a licence callable binding the CDM before the swap.
Gate a title on audio or video languages that must exist, independent of what -l/-al/-vl select. --require-subs joins the same gate: its --s-lang conflict and download-all side effect are gone, so -sl now decides what to keep. Requiring a language that is also excluded fails at parse time.
Renames 314 defs and 41 module/class-level constants. Keeps 11 names underscored where the stripped form collides in scope, plus 5.5.0 shims for the three symbols external service repos call.
Missing requested subtitle and audio languages now warn and continue when --best-available is set instead of hard-exiting. Without the flag, missing languages still produce an error and exit, matching the prior strict behavior. Audio missing-lang detection is now symmetric with subtitles.
- add find_missing_langs helper in core/utilities for reuse between s_lang and a_lang paths (skips all/best/orig sentinels)
- refactor dl.py s_lang/a_lang checks to share the helper
- add tests/lang_selection covering match primitives, helper output, and tricky langcodes corners (zh-Hans/zh-Hant/zh-CN/zh-TW/zh-HK, cmn/yue, fil/tl/tgl)
- clean up unused-var ruff F841 in tests/remote/unit/
Covers RemoteClient/RemoteService, REST routes, handlers, SessionStore, InputBridge, DownloadQueueManager, errors, compression, and serve CLI. E2e tier opts in via --live and can auto-spawn its own serve.
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