fix(portable): qualify merged Classic login packet source

[zoeyrose]

Summary

Pin the portable Classic consumer to the merged login and notification fixes from atrinik/classic#551.

Implementation / behavior

• Use merged Classic source d926f6fd0418fb1af9060158c43d8d3ff5252580 in portable/contract.json.
• Preserve the existing Debian baseline, canonical shader tools, source and license verification, and normal publication workflow.

Validation

The existing portable validation and real offline consumer jobs must pass for this exact producer head. Final local and CI evidence will be recorded before readiness.

Limitations / follow-up

Immutable image and platform digests become available after maintainer merge and successful normal semantic-release publication. Wrapper atrinik/atrinik#615 consumes those published identities.

Closes #50

Delivery validation

Reviewed head: 0b268a4beb6f3b847728fed16c47c1618be39050.

• Final-head local checks passed: 11 portable ABI tests, 21 dependency/shader/Vulkan tests, required-check aggregation tests, and git diff --check.
• Normal portable validation run passed Dockerfile checks, the non-root baseline smoke, and all 83 offline Classic consumer tests. The retained artifact confirms Classic d926f6fd0418fb1af9060158c43d8d3ff5252580, glibc 2.36, and generic x86-64 compiler flags across 274 compile units.
• Independent whole-diff review found zero actionable issues. CodeQL, PR-title policy, and the required aggregate passed. Unaffected Linux, Classic CI, and Windows image jobs were skipped by the existing path selector.
• This is CPU-only build/consumer evidence. It does not establish hardware GPU gameplay qualification.

The published image index/platform digests remain pending maintainer merge and the normal semantic-release publication workflow. No manual release or registry publication was performed.

[github-actions]

🎉 This PR is included in version 1.13.0 🎉

The release is available on:

• v1.13.0
• GitHub release

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