chore: prepare v0.5.1 release

[snkmcb]

Prepares the repository for the v0.5.1 release.

Summary:

• Finalize CHANGELOG.md and synchronize all declared versions to 0.5.1.
• Document manual release artifact checksum, manifest, SBOM, and ABI verification.
• Record hosted Windows L4, macOS arm64 L5, and Linux L5 evidence in the capability and configuration references.

Verification:

• ctest: 13/13 passed.
• v0.5.1 aggregate package generated twice with identical digest.
• Package-origin verification: gaussian-ply, gaussian-sog, and gaussian-spz each 15 pass, 0 fail, 1 skip.
• scripts/release.py guard --tag v0.5.1 passed.
• Hosted release dry-run #34996154493 passed, including aggregate assembly.

After merge, push tag v0.5.1 to run the draft release workflow. Add docs/releases/v0.5.1.md only after the tag and published artifact checksums exist, per the repository release policy.