mam: do not file another series volume from an unsnatched row

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Bug and impact

With mam/accept_unsnatched on, a series volume whose title does not write "Book N" (the number lives only in id3 SERIES/PART, or in a parsed folder such as Domagoj Kurmaic - Mother of Learning, Book 2) was accepted against another volume of the same title with a different MAM series_info part. The audio of book 1 could be filed as book 2.

samePosition already refused a title-written Book 2 against a part-3 row. It returned True whenever both titles lacked Book N, so the series parts were never compared.

Root cause

samePosition treated "neither title writes Book N" as no conflict. applyParsedName copies title/authors, not series/part, so a junk-tag file whose folder carried the volume still had nothing for samePosition to compare.

Fix

• When neither title writes Book N but both books have a series part, require those parts to overlap.
• In pickUnsnatched, apply the parsed folder part when the tags have none.
• A row with no series_info is still accepted (cannot refuse the release's own torrent if MAM omitted the part).

Validation

• GateTest.test_series_parts_must_agree_when_neither_title_writes_book_n: part 1 vs part 2 is refused; matching parts and 2/2.0 still accept.
• GateTest.test_parsed_folder_part_is_used_when_id3_has_no_series_part: junk tags + Author - Title, Book 2 refuse MAM part 1/3 and accept part 2.
• Existing Mother of Learning / Leviathan Wakes position tests still pass.
• Full suite: 264 tests OK.