Improve full bleed support

[hatton]

Implements full-bleed behavior so source page box intent is preserved through passthrough and imposition flows.

Key changes:

• Keep crop marks as an outer-margin concern (MediaBox growth) without changing intended trim/bleed geometry.

• Update NullLayoutMethod to preserve source TrimBox/BleedBox and only offset boxes when crop marks are enabled.

• Update SideFoldBookletLayouter to map source TrimBox to panel trim and clip drawing to mapped bleed bounds.

• Fix right-edge crop-mark coordinate placement bug in DrawCropMarks.

• Expand README Full Bleed documentation and clarify client responsibility for fold-edge behavior.

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This change is 

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 8 additional findings.

[hatton]

[GPT-5.3-Codex] Addressed both unresolved RTL review findings.

1. src/LayoutMethods/SideFold4UpBookletLayouter.cs (thread: Improve full bleed support #3 (comment))
   Fixed in commit 6d81303 by restoring _rightToLeft-aware box selection in DrawInferiorSide and DrawSuperiorSide, so superior/inferior page placement swaps correctly for RTL.

2. src/LayoutMethods/SideFold4UpSingleBookletLayouter.cs (thread: Improve full bleed support #3 (comment))
   Fixed in commit 6d81303 by restoring _rightToLeft-aware box selection in all four corner draw methods.

Validation run:
dotnet test tests/sillsdev.dotImpose.csproj -c Debug -> 41 passed, 0 failed.