feat: DXF attachment anchoring, stable attachment switching, and empty-line layout bounds

[mlightcad]

Summary

• Adds initialAttachmentPoint (DXF group 71) to MTextInputBoxOptions and exposes getMTextAttachmentPoint() so callers can align editor creation with MTEXT attachment semantics.
• Positions the editor’s MTEXT column using attachment-aware local X (attachmentColumnMinLocal), consistent with mtext-renderer / MText.calculateAnchorPoint frame conventions (left / center / right columns and baseline variants).
• When the ribbon attachment changes, adjusts position (and cursor view transform) so the on-screen text frame stays fixed while the insertion anchor updates.
• Improves vertical chrome bounds by unioning the layout container with empty logical cursor lines only, preserving trailing blank rows without reintroducing oversized renderer line strips on non-empty rows.
• Bumps @mlightcad/mtext-input-box to 0.2.10.
• Consolidates the Vitest @mlightcad/mtext-renderer mock (removes duplicate vi.mock) and adds coverage for top-right column offset and unionLayoutContainerWithCursorLines behavior.