RFC 9110 clamps a range end past the resource to its last byte; the guard
rejected that valid response. The descriptive-audio filter also had no
emptiness check, so a title with only described audio failed with a
misleading missing-language error.
Submitting every segment up front convoyed on ThreadPoolExecutor's process-global lock, starving the event drain so per-track progress bars only advanced once the download had finished.
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.
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