--color transparent is accepted and listed as a known name, but produces solid black icons #158

What's wrong

transparent is one of the accepted colour names. The README lists it ("Choosing a Colour", line 111), and the validation error message prints it through ColorParser.KnownNames.

IconHelper deliberately discards the colour's alpha, because the output alpha carries the coverage. NamedColors.Transparent is #00000000, so its RGB is 0,0,0. Every icon therefore comes out as an opaque black coverage mask, and the tool exits 0. A user who asked for "transparent" gets the opposite of what the word means, and nothing tells them.

Evidence

• IconHelper/ColorParser.cs:30: NamedColors.TryGet(trimmed, out color) accepts transparent.
• In ktsu.Semantics.Color, NamedColors.Transparent is Color.FromHex("#00000000").
• IconHelper/IconHelper.cs:224: (byte colorR, byte colorG, byte colorB, byte _) = color.ToBytes(); drops the alpha.
• IconHelper.Test/ColorParserTests.cs:87 asserts that KnownNames contains "transparent".

Suggested fix

• Reject transparent in ColorParser.TryParse or Arguments.Validate. The message should explain that the output alpha is the coverage, so a fully transparent colour has no meaning here.
• Drop transparent from KnownNames, and update the README list and the test to match.
• Optional: also reject any 8-digit hex whose alpha is 00. The README already says that alpha is ignored.

Acceptance criteria

• iconhelper -c transparent … exits 1 with a clear message.
• transparent no longer appears in the known-names list or in the README.