feat: add SHA256 verification for direct downloads

Store SHA256 hashes when running update scripts and verify them
when downloading files at build time. This improves security by
detecting unexpected changes to downloaded artifacts.

Platforms covered: macOS (CPython, PyPy, GraalPy), iOS, Android,
virtualenv, and python-build-standalone. Windows (nuget) and
Linux (Docker) are excluded.

SHA256 sources per platform:
- macOS/iOS/Android CPython (python.org): sha256_sum from API
- GraalPy: .sha256 sidecar assets from GitHub releases
- python-build-standalone: SHA256SUMS file in release
- PyPy, BeeWare iOS, Maven (Chaquopy): stream-download and compute

Changes:
- cibuildwheel/util/file.py: add sha256 param to download()
- cibuildwheel/platforms/{macos,ios,android}.py: add sha256 to
  PythonConfiguration and pass to download()
- cibuildwheel/venv.py: read sha256 from toml and pass to download()
- cibuildwheel/util/python_build_standalone.py: add sha256 to
  PythonBuildStandaloneAsset and pass to download()
- cibuildwheel/resources/build-platforms.toml: add sha256 fields
- cibuildwheel/resources/virtualenv.toml: add sha256 field
- cibuildwheel/resources/python-build-standalone-releases.json: add sha256
- bin/update_pythons.py: compute/store sha256 per source strategy
- bin/update_virtualenv.py: compute sha256 by streaming download
- bin/update_python_build_standalone.py: parse SHA256SUMS file

Closes #908

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Assisted-by: copilot-cli:claude-sonnet-4.6

bin/update_virtualenv.py

@@ -4,6 +4,7 @@
 # dependencies = [
 #   "click",
 #   "packaging",
+#   "requests",
 #   "rich",
 #   "cibuildwheel",
 # ]
@@ -15,12 +16,14 @@
 
 import dataclasses
 import difflib
+import hashlib
 import logging
 import tomllib
 from pathlib import Path
 from typing import Final
 
 import click
+import requests
 import rich
 from packaging.version import Version
 from rich.logging import RichHandler
@@ -92,16 +95,26 @@ def update_virtualenv(force: bool, level: str) -> None:
     if latest_release.version > Version(local_version):
         version = latest_release.name
         url = latest_release.download_url
+        # Compute sha256 by streaming the new download
+        log.info("Computing sha256 for %s...", url)
+        response = requests.get(url, stream=True)
+        response.raise_for_status()
+        hasher = hashlib.sha256()
+        for chunk in response.iter_content(65536):
+            hasher.update(chunk)
+        sha256 = hasher.hexdigest()
     else:
         version = local_version
         url = default["url"]
+        sha256 = default.get("sha256", "")
 
     configurations["default"] = {
         "version": version,
         "url": url,
+        "sha256": sha256,
     }
     result_toml = "".join(
-        f'{key} = {{ version = "{value["version"]}", url = "{value["url"]}" }}\n'
+        f'{key} = {{ version = "{value["version"]}", url = "{value["url"]}", sha256 = "{value.get("sha256", "")}" }}\n'
         for key, value in configurations.items()
     )