fix(dom): preserve model-declared blank pages

[seonghobae]

Summary

Preserve model-declared blank pages when other pages contain indexed content. The shared DOM builder previously iterated only content indices, so a three-page document with a blank middle page returned pages 1 and 3.

This PR is stacked on #822 at 927ff2dec160a00ed11ec045727a422ff7ba4cbd to preserve the existing security owner changes. Its unique delta is two production lines and one synthetic regression test. Merge after #822 and retarget to develop; do not bypass its review/security gates.

Verification

• Regression failed before the fix: expected [1, 2, 3], received [1, 3].
• Full suite: 487 passed in 12.04s, warnings as errors; production statement and branch coverage 100%.
• Ruff and git diff --check passed.
• Tests ran from this worktree with PYTHONPATH=src using the existing Python 3.13.15 supplier virtual environment. No dependency installation or lock changes.

The test preserves blank-page dimensions and surrounding content. Actual GPU OCR, MinerU 4 migration, model quality, and deployment remain unverified; this repairs the existing shared DOM contract independently.

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[seonghobae]

Exact-head admission correction — 65696f393224a1e08ab42fff5151cbf3639a8a4e

Ready is review admission only. Fresh audit against base 927ff2dec160a00ed11ec045727a422ff7ba4cbd found:

• mergeability=false on current base

This PR is moved to Draft/Proposed until the causal owner repair is present on a successor exact head and re-audited. Queued/pending work is neither an additional blocker nor passing evidence. No Close, force push, destructive rebase, manual rerun, synthetic status/approval, merge, auto-merge, or bypass was performed.