Optimize DecodePubCurveKey and fix off-by-one

[mtmk]

Optimized allocations of DecodePubCurveKey also it returned 33 bytes instead of 32, including a stray CRC byte in the Curve25519 public key material. Worked since TweetNaCl only reads the first 32 bytes, but diverged from the Go reference implementation.

• Fix slice to use CurveKeyLen (32) matching Go's raw[1:end]
• Optimize: reduce allocations
• More tests covering with extensive coverage of DecodePubCurveKey

[Copilot]

Pull request overview

This PR improves Curve public key decoding in KeyPair to reduce allocations and make the decoded output consistently 32 bytes, and updates the test setup to validate the behavior and rely on internals access rather than compiling library sources into the test project.

Changes:

• Refactors DecodePubCurveKey to use Span/stackalloc and adds an overload that decodes into a caller-provided buffer.
• Adds extensive unit tests covering decoding correctness, CRC validation, and seal/open round-trips.
• Simplifies the test project by removing direct compilation of library internal sources and enabling access via InternalsVisibleTo.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
NATS.NKeys/NATS.NKeys.csproj	Grants internals access to test assemblies via InternalsVisibleTo.
NATS.NKeys/KeyPair.cs	Reworks curve public key decoding to be allocation-light and returns exactly 32 bytes.
NATS.NKeys.Tests/NKeysTest.cs	Adds targeted tests for curve public key decoding and integration scenarios.
NATS.NKeys.Tests/NATS.NKeys.Tests.csproj	Removes compiled-in library sources now that internals are visible from the main assembly.

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[scottf]

LGTM

[Jarema]

LGTM