fix(secp256r1): bind discriminator, auth_payload, and payer to signature hash

Fixes #8: Include instruction discriminator in signed_payload hash to prevent
cross-instruction signature replay attacks.

Fixes #9: Bind signature to payer (on-chain signer) by including payer.key()
in the hash calculation and requiring payer to be a signer.

Changes:
- mod.rs: Updated sol_sha256 to include discriminator, auth_payload, and payer
- introspection.rs: Fixed precompile layout offsets (signature-first, 33-byte key)
- tests-rpc/: Added RPC integration test for Secp256r1 verification

The hash now includes:
1. discriminator (instruction type binding)
2. auth_payload (RP ID, flags binding)
3. signed_payload (original payload)
4. slot (replay protection)
5. payer.key() (signer binding)

Author: onspeedhp

Cargo.lock

@@ -1,6 +1,6 @@
 [workspace]
 resolver = "2"
-members = ["program", "no-padding", "assertions", "tests-e2e"]
+members = ["program", "no-padding", "assertions"]
 
 [workspace.dependencies]
 pinocchio = { version = "0.9", features = ["std"] }