Fix DESFire access rights packing order

[c3rebro]

Motivation

• The DESFire access-rights packing used by the library did not follow the datasheet nibble order and produced incorrect permission words on cards.
• The datasheet requires nibble order: Read (bits 15–12), Write (11–8), Read&Write (7–4), Change (3–0).
• The change corrects the packing/unpacking to match the spec and prevents producing reversed access-rights words.
• This is a breaking change for consumers that previously compensated for the reversed order.

Description

• Added ToAccessRightsWord() and FromAccessRightsWord(ushort) on DESFireFileAccessRights to pack/unpack using the datasheet nibble order.
• Replaced ad-hoc manual bit packing/unpacking in TWN4ReaderDevice.MifareDesfire.cs with the new helper methods (used for create/change/unpack of file access rights).
• Added unit tests tests/Elatec.NET.Tests/AccessRightsTests.cs to assert correct pack/unpack behavior.
• Kept public property names (ReadKeyNo, WriteKeyNo, ReadWriteKeyNo, ChangeKeyNo) unchanged for minimal surface-area impact, but behavior of packing is now spec-compliant.

Testing

• Ran dotnet test tests/Elatec.NET.Tests/Elatec.NET.Tests.csproj and the test run completed successfully.
• All tests passed: Passed! - Failed: 0, Passed: 23, Skipped: 0, Total: 23.
• Project builds during the test run with no test failures.
• New tests cover ToAccessRightsWord and FromAccessRightsWord packing/unpacking behavior.

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Codex Task