CreditCardSource never produced a Discover card

[holonsoftFounder]

Found while checking whether "all bugs gone" is actually true before tagging v6.0.0. It was not.

The bug

CreditCardType.Random picked the card type with Random.Next(1, 4), whose upper bound is exclusive, so it stopped at AmericanExpress. Discover = 4 is the last value of the enum and could never be produced. Measured over 5000 generated cards:

prefix 3 (AmericanExpress): 1640
prefix 4 (Visa):            1650
prefix 5 (MasterCard):      1710
prefix 6 (Discover):           0

The picker reads the selectable types from the enum now, so a card type added later is drawn without anyone having to remember this one line. The sequence of CreditCardSource changed, which is why this has to land before the tag rather than in 6.1.

How it was found

Two reflection sweeps over the built library, since reading code has repeatedly missed this class of bug:

1. Every public source (81 of them) instantiated with its parameterless constructor, 60 draws each, flagged for constant output, exceptions, hangs and infinity/NaN. Clean apart from the intentionally constant Default* and LoremIpsum sources.
2. Every source with a (min, max) constructor given a narrow range, checked that all 400 draws land inside it and that both bounds are reachable, plus a reversed range to check it throws instead of hanging or returning nonsense. Clean.

Neither sweep found anything else. The CreditCardSource bug came from a targeted grep for the off-by-one pattern that had already bitten RandomStringSource and UrlSource in #10.

Not fixed here

LoremIpsumSource(count) puts its blank-line separator in the wrong place: count: 3 gives TT\n\nT instead of T\n\nT\n\nT. It is deterministic text, so no generated sequence depends on it and 6.1 can fix it without breaking anyone.

Verification

dotnet build -c Release zero warnings, 867 tests per framework on net8/9/10, 2607 total, all green.

🤖 Generated with Claude Code