fix(metadata): order series positions with an invariant parse

[m4bard]

Fixes #795.

A series position arrives from Audible's sequence/sort field as a string that always uses . as the decimal separator. ParseSeriesPosition parsed it with the ambient culture, so on a server whose culture treats . as the group separator the value was read as a different number.

de-DE   "1.5" parses as 15          novella sorts after book 10
fr-FR   "1.5" does not parse at all  falls to decimal.MaxValue, sorts last

A default container runs under the invariant culture and was never affected, which is why this is easy to miss. It shows up once the process has a real culture, which happens when LANG or LC_ALL is set and on a desktop install that inherits the operating system's locale.

The parse is now pinned to CultureInfo.InvariantCulture with NumberStyles.Number. Sort data, parsed the same way everywhere.

The tiebreak question from the issue, deliberately not answered here

I asked on the issue what should happen to a position that is not a decimal at all, the 1-4 of an omnibus. This PR does not change that: those still sort last, exactly as they do today.

That is not me deciding the question. It is me keeping this change to the part that is unambiguously a bug, so the ordering of omnibus entries stays whatever you decide it should be, separately. If you would rather they sorted first, or kept their catalogue order, that is a different change and I am happy to write it once you say which.

Tests

SeriesPositionOrderingTests, thirteen cases. The parse is asserted directly under de-DE, fr-FR and the invariant culture, and the ordering is asserted end to end over a mixed series.

Control, with only the parse reverted to the culture-dependent form and everything else left in place:

Expected: 1,5                       Actual: 15
Expected: 1,5                       Actual: 79228162514264337593543950335
Expected: ["1", "1.5", "2", "10", "1-4"]
Actual:   ["1", "2", "10", "1.5", "1-4"]

That last line is the reported symptom: the novella lands after book 10.

ParseSeriesPosition goes from private to internal so the culture behaviour can be asserted on the method rather than inferred from the ordering. That accessibility change is why a plain revert of the whole commit will not compile; the control above reverts only the parse.

Full suite 3051 passed, 0 failed, 127 skipped, on canary d25b3e11 (1.3.3).

One housekeeping commit alongside: this branch predated TestClasses_FollowRepositoryConventions, so the test class needed BaseTests and its traits to satisfy the architecture suite.