Security: upgrade Sigstore trust stack past affected cryptography

[sftimeless]

Summary

• pin the reviewed Sigstore 4.5.0 trust stack with cryptography 50.0.0
• adapt Rekor evidence extraction to Sigstore 4's standardized bundle JSON model
• preserve offline verification of the historical Rekor v1 fixture and explicit inclusion-proof/checkpoint checks
• prepare the changed verifier bytes under the new, unpublished 3.24.2 release-candidate identity
• regenerate the embedded v3.24.2 release manifest and bind its expected signer to refs/tags/v3.24.2

Local validation

• Python 3.12 full verifier suite: 1,004 passed, 38 skipped, 5 xfailed
• focused trust/release suite: 93 passed, 22 skipped
• version/release-manifest regression suite: 35 passed
• clean Python 3.12 wheel and sdist build; Twine and wheel metadata checks passed
• installed-wheel CLI, historical artifact, tamper rejection, Sigstore/Rekor/TSA, pip check, and runtime audit passed
• candidate online self-check fails honestly at the missing v3.24.2 release fetch and does not fall back to v3.24.1
• independent --published-wheel 3.24.1 self-check passes for the genuine historical release

Release boundary

3.24.2 is a release candidate only. This PR does not tag, sign, publish, or claim that v3.24.2 release artifacts exist. After an explicitly authorized merge, v3.24.2 must be tagged on the exact final merge commit and produced by the approved signed release workflow.

State

Local/non-production dependency remediation only. This draft does not merge, tag, publish, deploy, access production, change production configuration, provision secrets, or alter historical data.