[hakula139]

Summary

Overlay and Home Manager consumers now reuse the flake's upstream pinned package, allowing Cachix substitution when the caller uses a different nixpkgs revision.

• Package identity: select the upstream package output on supported flake systems and preserve source construction on existing overlay-only platforms.
• Integration: retain the source builder and all package outputs, update the existing identity checks, and remove obsolete caller-toolchain workarounds from the installation guide.

Closes #802.

Test plan

• Commit hooks passed. Overlay and Home Manager checks passed on all three systems, and the native Darwin Home Manager check built with the final caller-package fixture.
• Old-behavior probes failed for overlay and Home Manager caller-side reconstruction. The existing x86_64 Darwin overlay fallback still selects the original builder.
• The full repository verification block and native Darwin nix flake check passed before the final test-only fixture correction. That correction passed targeted checks and commit hooks. Cache substitution and release pin publication remain unexecuted.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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