BIP352: complete type annotations on bitcoin_utils class methods

[omipheo]

Summary

Direct follow-up to PR #2154 (which annotated the free-function half of bip-0352/bitcoin_utils.py) and 2f7117c ("BIP352: fix Any typing"). The four classes in this file — COutPoint, VinInfo, CScriptWitness, and CTxInWitness — still had unannotated
__init__, serialize, deserialize, and is_null methods, so mypy could not flow types through the surrounding reference.py
code that constructs and passes these objects.

This PR annotates every method on all four classes:

Class	Methods annotated
COutPoint	__init__(hash: bytes, n: int) -> None, serialize() -> bytes, deserialize(f: BytesIO) -> None
VinInfo	__init__ (typed Optional defaults for the three construct-on-None fields)
CScriptWitness	__init__() -> None, is_null() -> bool, plus an inline stack: List[bytes] declaration matching what the rest of the file already assumes
CTxInWitness	__init__() -> None, deserialize(f: BytesIO) -> "CTxInWitness" (forward ref since it returns self), is_null() -> bool

Optional is added to the existing typing import; List was already added by #2154. No behavior changes.

Verification

$ mypy --ignore-missing-imports ./bip-0352/bitcoin_utils.py
Success: no issues found in 1 source file

$ python3 -c "
import sys
sys.path.insert(0, 'bip-0352')
sys.path.insert(0, 'bip-0352/secp256k1lab/src')
import bitcoin_utils
from io import BytesIO

op1 = bitcoin_utils.COutPoint(b'\\x11' * 32, 7)
data = op1.serialize()
print('COutPoint serialize len:', len(data))
op2 = bitcoin_utils.COutPoint()
op2.deserialize(BytesIO(data))
print('COutPoint round-trip:', op2.hash.hex()[:16], op2.n)

csw = bitcoin_utils.CScriptWitness()
print('CScriptWitness empty is_null:', csw.is_null())
csw.stack.append(b'\\x01\\x02')
print('CScriptWitness with-stack is_null:', csw.is_null())

ctw = bitcoin_utils.CTxInWitness()
print('CTxInWitness deserialize returned self:',
      ctw.deserialize(BytesIO(b'\\x00')) is ctw)

vi = bitcoin_utils.VinInfo()
print('VinInfo defaults:', type(vi.outpoint).__name__,
      type(vi.txinwitness).__name__, type(vi.private_key).__name__)
"
COutPoint serialize len: 36
COutPoint round-trip: 1111111111111111 7
CScriptWitness empty is_null: True
CScriptWitness with-stack is_null: False
CTxInWitness deserialize returned self: True
VinInfo defaults: COutPoint CTxInWitness Scalar

$ cd bip-0352 && python3 -c "import reference" (exits 0 — reference.py still imports cleanly)

Notes

• Direct follow-up to BIP352: complete return type annotations in bitcoin_utils #2154 ("BIP352: complete return type annotations in bitcoin_utils") in the same file. Closes the remaining annotation gap.
• No behavior changes. No new dependencies.
• Single-commit, single-file diff: bip-0352/bitcoin_utils.py, +18 / −11.