fix(viewer): use batch geometry bounds instead of EXTMIN/EXTMAX for open zoom

[mlightcad]

Summary

• Remove provisional zoom from database EXTMIN/EXTMAX at document open; real-world DXFs often carry stale header extents that diverge from rendered geometry.
• Simplify progressive open fit to always derive framing from batch-converted geometry bounds (incremental fit for large drawings, final fit via zoomToFitDrawing).
• Drop resolveModelSpaceExtentsBox and related skip-final-fit logic tied to initial extents.

Test plan

• Open a DXF/DWG with stale or outlier EXTMIN/EXTMAX header values and confirm the view frames rendered geometry, not unrelated coordinates.
• Open a small drawing with progressiveRendering enabled and verify final zoom-to-fit is correct.
• Open a large drawing with progressiveRendering enabled and verify incremental framing during conversion, then final fit when complete.
• Open into paper space with layout limits and confirm layout-based zoom still works.
• Pan/zoom manually during progressive open and confirm auto-framing stops.

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