feat: add .NET Framework 4.8 (net48) support

[daviddesmet]

What

Re-adds net48 as a target framework for Paseto.Core (now net48;net8.0;net10.0), so the library can be consumed from legacy .NET Framework 4.8 apps. Closes #.

Why

A user asked whether net48 could be supported. It can — net48 was targeted before and dropped around 78ee106. Both crypto dependencies (NaCl.Core, BouncyCastle.Cryptography) support netstandard2.0, so they run on net48.

How

Six BCL crypto/encoding APIs used by the library don't exist on net48 and are reimplemented behind #if NETFRAMEWORK, backed by BouncyCastle (already a dependency):

Modern API	net48 replacement
HKDF.DeriveKey	HkdfBytesGenerator (Internal/Hkdf.cs)
Rfc2898DeriveBytes.Pbkdf2	Pkcs5S2ParametersGenerator (Internal/Pbkdf2.cs)
RandomNumberGenerator.Fill/GetBytes	RandomNumberGenerator.Create() (Internal/Rng.cs)
CryptographicOperations.FixedTimeEquals	constant-time bit-diff loop (Internal/CryptoBytes.cs)
CryptographicOperations.ZeroMemory	Array.Clear under [NoInlining|NoOptimization]
RSA.ExportRSAPrivateKey/PublicKey	BouncyCastle PKCS#1 export (Protocol/Version1.cs)

Plus compiler/BCL polyfills for net48: IsExternalInit (init setters), RuntimeHelpers.GetSubArray (array range slicing), NotNullAttribute, Guard.NotNull, and span/HashData fallbacks in Base64UrlEncoder, EncodingHelper, and PaserkSeal.

net48-only packages: System.Memory, IndexRange, System.Text.Json, and Microsoft.NETFramework.ReferenceAssemblies (lets net48 compile on Linux/macOS CI runners). The test project also multi-targets net48 (ReadKey PEM handling rewritten with BouncyCastle), and build.cake runs the net48 test suite on the Windows runner — the runtime security gate.

Verification

• ✅ net8 suite: 521/521 pass — refactor is regression-free
• ✅ Source + tests build across all three TFMs in Release
• ✅ Every net48 crypto shim proven byte-identical to the BCL API it replaces (HKDF-SHA384, PBKDF2-SHA384, constant-time equals, GetSubArray, RSA PKCS#1 keygen/export, and ReadKey for EC / RSA-private / RSA-public, checked against the real PASETO test-vector PEMs)
• ⚠️ net48 tests execute only on Windows (no .NET Framework runtime on Linux/macOS); the CI Windows runner is the gate. Byte-equivalence was verified locally to de-risk that run

Closes #99

[daviddesmet]

⚠️ Need to test on Windows before merging.

[daviddesmet]

Dropped xunit.categories, swapped the 2 [Category("CI")] → built-in [Trait("Category", "CI")], removed the stale NoWarn, regenerated the lock file.

Cause: Paseto.Tests is strong-named (InternalsVisibleTo public key), but xunit.categories isn't strong-named → net48's AppDomain discovery refuses to load it

[codecov]

Codecov Report

❌ Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.81%. Comparing base (176d8b1) to head (5fff226).

Files with missing lines	Patch %	Lines
src/Paseto/Internal/CryptoBytes.cs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
- Coverage   90.82%   90.81%   -0.01%     
==========================================
  Files         108      112       +4     
  Lines        5580     5588       +8     
  Branches      363      363              
==========================================
+ Hits         5068     5075       +7     
- Misses        512      513       +1     

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