Prevents cryptic trait conflict errors by detecting incompatible feature
combinations at compile time with clear error messages.

Changes:
- Add compile_error! macros for conflicting public features
- Configure docs.rs to avoid --all-features (use default features)
- Document architectural design decisions in Cargo.toml

The PASETO specification recommends using a single version per application.
Multiple public features cause conflicting From<ed25519_dalek::ed25519::Error>
implementations. These checks enforce this design at compile time.

Error messages guide users to:
- Choose only one public feature
- Reference issue #48 for context
- Understand this is intentional design

Docs.rs configuration prevents build failures by using default features
(batteries_included + v4_local + v4_public) instead of --all-features.

Addresses #48

Update rand_core dependency to latest compatible version.

All 8 feature combinations tested and passing:
- v1_local: 12 tests passed
- v2_local: 14 tests passed
- v3_local: 16 tests passed
- v4_local: 13 tests passed
- v1_public: 8 tests passed
- v2_public: 10 tests passed
- v3_public: 7 tests passed
- v4_public: 10 tests passed

Update thiserror dependency from 1.0.69 to 2.0.17. All tests pass
across the entire feature matrix (v1-v4, local and public).

Changes include automatic clippy fixes for code style issues.

Update primes from 0.3 to 0.4 in dev dependencies.

Resolve clippy::doc-lazy-continuation warnings by properly indenting
continuation lines in documentation comments.

Remove redundant parentheses from impl trait parameter types in
function signatures for all PASETO version implementations.

Replace ring::constant_time::verify_slices_are_equal with subtle crate's
ConstantTimeEq trait for constant-time byte comparison. The subtle crate
is already present as a transitive dependency and provides the same
security guarantees.

Add subtle 2.6 as a direct dependency for constant-time operations.
While already present as a transitive dependency, explicit declaration
is required to import and use its ConstantTimeEq trait.

Use PasetoError::Cryption for MAC tag comparison failures instead of
non-existent InvalidToken variant. This maintains security by providing
a general unspecified cryptographic error.

Use bool::from() for explicit type conversion of subtle::Choice to bool
to satisfy type inference requirements.