Y2B-205 — Enforce D08 layout before packaging

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Depends on Y2B-201 (#8), D08/Y2E-130. Offline implementation only.

Implement fail-closed arithmetic for the exact D08 relocation, memory, initramfs and LK staging constraints. Keep the policy constants fixed; no automatic RAM expansion. Expose an importable validator and machine-readable intervals. Subsequent artifact checkpoints will feed real ELF symbols, decoded DT and package bytes into it.

Acceptance:

Check every D08 size cap, alignment, positive relocation delta, collision branch, live workspace/output overlap, surviving DTB/kernel overlap and LK staging/partition bound.
Explicit checked 32-bit physical-address arithmetic and reject missing/invalid inputs.
Meaningful negative tests cover independent overflow, size, alignment, BSS, workspace and overlap failures; a bounded valid fixture passes.
Document that arithmetic acceptance is neither hardware safety nor artifact validation without real ELF/FDT/package evidence.

Validation: four unit tests, including 17 independent invalid-layout cases, pass. Real ELF/FDT/package validation follows in the artifact checkpoints; this checkpoint alone does not accept a BOOTIMG.