Implements signed index support

[zooba]

Fixes #308

[Copilot]

Pull request overview

This PR adds signed-index verification for runtime feed JSONs, aiming to detect tampering of the python.org index while keeping the feature configurable for other sources (Fixes #308).

Changes:

• Introduces a Windows native _native.verify_trust implementation to validate a JSON file against a signed catalog (.cat) using OS trust.
• Extends IndexDownloader to fetch and validate {index_url}.cat before processing indexes, with configurable per-source requirements (source_settings).
• Updates CI pipelines to generate/sign catalog files for partitioned indexes and upload the resulting .json.cat artifacts; adds tests and test fixtures.

Reviewed changes

Copilot reviewed 18 out of 22 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
tests/test_verify.py	Adds end-to-end tests covering successful verification and failure/bypass paths.
tests/data/self-signed.cat	Adds an untrusted (self-signed) catalog fixture for negative tests.
tests/data/psf-signed.cat	Adds a trusted/signed catalog fixture for positive tests.
tests/data/index-windows.json	Adds a test index fixture used by verification tests.
src/pymanager.json	Adds default source_settings for python.org index URLs.
src/manage/urlutils.py	Implements signature download/verification flow in IndexDownloader.
src/manage/list_command.py	Updates call sites for new IndexDownloader(cmd, ...) signature.
src/manage/install_command.py	Updates call sites for new IndexDownloader(cmd, ...) signature.
src/manage/indexutils.py	Extends index schema with signature-related fields.
src/manage/exceptions.py	Adds an exit code to InvalidFeedError for CLI handling.
src/manage/commands.py	Adds source_settings to config schema and command attributes.
src/manage/init.py	Treats InvalidFeedError as a user-facing error with an exit code.
src/_native/verify_trust.cpp	Adds native Windows verification logic for signed catalog + file hash membership.
scripts/repartition-index.py	Updates for new IndexDownloader constructor signature.
ci/upload.py	Uploads .json.cat alongside .json for index publishing.
ci/sign-files.yml	Adds reusable signing template used by pipelines.
ci/repartition-index.yml	Generates catalogs for index JSONs, signs them, and handles publish vs pretend-publish paths.
ci/release.yml	Refactors signing parameters and uses sign-files.yml; restructures post-test and publish stages.
_msbuild_test.py	Links wintrust.lib and exports verify_trust for test builds.
_msbuild.py	Links wintrust.lib and exports verify_trust for production builds.

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