Expose typed CheckSigFromStack builtins

[elldeeone]

Summary

Adds typed SilverScript builtins for Kaspa signature verification from stack:

• CheckSigFromStack(datasig signature, byte[32] digest, pubkey publicKey)
• CheckSigFromStackECDSA(datasig signature, byte[32] digest, byte[33] publicKey)

These expose the digest-level VM primitives directly, so contracts can choose the hashing scheme they need before passing a fixed 32-byte digest.

Changes

• Lower CheckSigFromStack to OP_CHECKSIGFROMSTACK.
• Lower CheckSigFromStackECDSA to OP_CHECKSIGFROMSTACKECDSA.
• Type-check builtin arguments and bool return values.
• Require a fixed byte[32] digest instead of accepting raw byte[] messages.
• Add runtime tests for valid and forged Schnorr/ECDSA signatures.
• Update examples, docs, and syntax highlighting.

Testing

• npm --prefix tree-sitter run sync:highlights
• cargo test -p silverscript-lang checksig -- --nocapture
• cargo test -p silverscript-lang
• cargo fmt --all --check
• git diff --check