Raise ValueError from verify_digest for Edwards curve keys (fixes #349)

[apoorvdarshan]

Fixes #349

Root cause

VerifyingKey.verify_digest is missing the CurveEdTw guard that
VerifyingKey.verify and the SigningKey.sign_digest* methods already
have. For an Edwards-curve (EdDSA) key the method falls through to the
ECDSA code path and reaches:

r, s = sigdecode(signature, self.pubkey.order)

An EdDSA public key's pubkey is a PublicKey object that has no
order attribute, so the call raises a confusing AttributeError
instead of a clear "unsupported for Edwards curves" error. This is
exactly the behaviour reported in the issue.

Reproduction

from ecdsa.keys import SigningKey, VerifyingKey
from ecdsa.curves import Ed25519

sk = SigningKey.generate(curve=Ed25519)
vk = sk.verifying_key
msg = b"message"
sig = sk.sign(msg)

print("verify(sig, data) ->", vk.verify(sig, msg))   # correct usage
vk.verify_digest(sig, msg)                            # the bug

Before the fix:

verify(sig, data) -> True
Traceback (most recent call last):
  ...
  File ".../ecdsa/keys.py", line 735, in verify_digest
    r, s = sigdecode(signature, self.pubkey.order)
AttributeError: 'PublicKey' object has no attribute 'order'

After the fix:

verify(sig, data) -> True
ValueError: Method unsupported for Edwards curves

Fix

Add the same two-line guard used by sign_digest() /
sign_digest_deterministic() / sign_number() at the top of
verify_digest():

if isinstance(self.curve.curve, CurveEdTw):
    raise ValueError("Method unsupported for Edwards curves")

and document the raised exception in the docstring. EdDSA signs the
message directly and has no pre-hashed-digest verification API, so
verify() is the correct entry point (as noted in the issue). The
change is fully localized to verify_digest; no other code path is
affected.

Tests

Added test_ed25519_verify_digest to TestVerifyingKeyFromDer in
src/ecdsa/test_keys.py, asserting that verify_digest on an Ed25519
key raises ValueError containing "Method unsupported for Edwards".
It mirrors the existing test_ed25519_sign_digest style. Confirmed it
fails on unmodified master (with the AttributeError above) and
passes after the fix.

Test runs (Python 3.14, pytest):

• test_keys.py: 179 passed
• test_eddsa.py: 87 passed
• test_pyecdsa.py: 800 passed
• test_malformed_sigs.py: 205 passed

flake8 setup.py speed.py src is clean, and black --line-length 79
reports both changed files unchanged.

Disclosure: prepared with AI assistance; reviewed and verified locally.