Declare intrinsic banner dimensions

[vartaxe]

Summary

Add the banner SVG's intrinsic height="320" beside width="1280" on both landing pages to prevent layout shift, strengthen the existing branding regression to require both dimensions, and generate the exact-byte CHECKSUMS.txt manifest.

Safety impact

• The fail-closed target-selection and pre-clean guards are unchanged; no runtime PowerShell implementation changed.
• No support boundary, destructive behavior, or task-sequence requirement changed.
• No sensitive deployment information is included.

Validation

• powershell.exe -NoProfile -ExecutionPolicy Bypass -File .\build\Invoke-Validation.ps1 passes — 63 passed, 0 failed, 0 skipped, 0 not run.
• Root and packaged script copies remain byte-identical — enforced by the passing validator.
• New or changed logic has synthetic tests that do not modify hardware — the existing branding regression now requires width="1280" and height="320" on both landing pages.
• Documentation reflects changed parameters, integration, or compatibility — no runtime documentation change is needed; only image presentation metadata changed.
• Destructive behavior was tested only on disposable media, or the untested hardware boundary is stated explicitly — no destructive behavior changed or was exercised.

CHECKSUMS.txt contains 30 sorted uppercase SHA-256 entries and was independently verified against every maintained file except .git and the manifest itself.

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