Security patch

v0.1.8 supersedes v0.1.7 and closes the issues found by independent release-blocking review.

• Runs official release-policy retrieval in a killable worker process under a 2.5-second monotonic total deadline. Deadline expiry terminates and joins the worker, so DNS, TLS, redirect, or slow-drip reads cannot leave a thread or socket behind.
• Keeps the existing exact official HTTPS URL, GET-only request, response-size bound, strict JSON schema, and fail-open operational behavior.
• Trusts POSIX cache files only when the opened descriptor is a user-owned, non-symlink regular file without group/other write access.
• On Windows, opens the cache first with CreateFileW(FILE_FLAG_OPEN_REPARSE_POINT) and validates that same handle's reparse/regular-file state, final user-profile containment, owner SID, and DACL before reading.
• Introduces cache schema v2 while reading schema v1, preserving the last accepted policy across negative-cache entries and system-clock regression.
• Rejects pre-epoch, excessively future-dated, rolled-back, naive-time, or internally inconsistent policies/caches.
• Expands CI to Python 3.11–3.13 on Linux, Windows, and macOS. The native Windows installer/hook E2E remains on Python 3.13.

Verification

• pytest: 201 passed, 3 skipped, 106 subtests passed
• unittest: 204 passed, 3 skipped
• Ruff, compileall, lock, provenance, chart, build, and Twine checks passed
• Built wheel installed into a clean Python 3.11 environment; brainctl --version returned 0.1.8
• Benchmark artifacts record clean source commit ae55ce04a362d4c0dab25b09e31d24f8dd1d0b04

See CHANGELOG.md for details.