chore(verify): add verify_manifest.py — the SHA-256 ship-manifest producer

Self-contained (stdlib) build/check/--selftest tool that generates the
per-version {version}/verify.json (schema + platform->filename->sha256)
uploaded to ModelScope, against which auto_update.py verifies the installed
engine lib on startup. Vendored here so the plugin's release flow is
self-contained (a copy also lives in the engine repo's ship tooling).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>