Refactor, add error enum, and add program id

[samkim-crypto]

Summary of Changes

Cleaned up a few things in the ed25519-verify crate for basic improvements and also consistency with the secp256k1-verify library.

• Safety-net tests: Added unit tests cross-checking the hand-transcribed curve constants (ED25519_BASEPOINT_NEGATED_COMPRESSED, EDWARDS_IDENTITY_COMPRESSED) against curve25519-dalek, plus isolated coverage for is_small_order, multiply_by_8, and compute_challenge. Also removed a dead default-features = false in ed25519-program's Cargo.toml that had no effect.
• Constant consolidation & module split: Refactored the six curve/field constants previously scattered across verifier.rs, scalar.rs, and lib.rs into a single constants.rs, and moved basic point arithmetic logic in verifier.rs to a separate points.rs module.
• Introduced Ed25519VerifyError: Ed25519Verifier::verify_signature now returns Result<(), Ed25519VerifyError> (six variants covering canonical/small-order rejections, malformed encoding, and signature mismatch) instead of ProgramError, and solana-ed25519-verify no longer depends on solana-program-error at all. ed25519-program's process_instruction now explicitly maps every verification failure to ProgramError::InvalidInstructionData.
• Feature-gated the crate: Added verify and instruction Cargo features (both default-on), so a client that only needs to build CPI instructions can depend on instruction alone without pulling in solana-curve25519/solana-sha512-hasher.
• Declared a canonical program ID and switched to solana-address: Added declare_id!("ed2DNnfPh19L66ahBPivbPkf5H1nW82zWTYGMjuQk9L") in instruction.rs, exposing id()/ID, and migrated the crate (and ed25519-program's tests) from solana-pubkey::Pubkey to solana-address::Address.
• Updated README.md: Documented the new error type, Cargo features, and canonical program ID. The CU cost table and make cu-program were left untouched, pending separate compute-unit testing in a follow-up PR.