fix: collapse quote-table before-spacer against preceding paragraph (#78)

[forest6511]

Summary

• Closes Quote (table-rendered): before-spacer not collapsed against preceding heading/text paragraph #78 — follow-up to Quote (table-rendered): adjacent ::: blocks produce 2 spacer paragraphs and excessive vertical whitespace #76/fix: collapse adjacent quote-table spacers (#76) #77.
• The previous fix only collapsed against empty spacer paragraphs (IsBodySpacerParagraph required no runs). That left a visible blank line whenever the preceding body element was an H2/H3 rendered via AddHeadingSingleParagraph (the default path for headings without BorderExtent="text"), or a regular text paragraph that already supplied SpaceAfter.
• Generalise AddCollapsingBeforeSpacer: fold the requested before-spacing into the previous paragraph's SpacingBetweenLines.After whenever that After is positive, regardless of whether the paragraph contains runs. CSS-like margin-collapse, applied uniformly to table-table, heading-table, and text-table boundaries.
• IsBodySpacerParagraph becomes unnecessary and is removed.

Why this is safe

When the previous paragraph already has After > 0, the new logic only ever grows that gap (max(existing, desired)). It never shrinks an existing gap and never reaches into paragraphs that don't carry an explicit trailing gap. Heading after-spacers, table/quote after-spacers, and explicit body-paragraph SpaceAfter are all handled the same way.

When the previous paragraph has no SpacingBetweenLines or After == 0, behaviour is unchanged: a fresh spacer paragraph is appended.

Test plan

• Existing 323 tests still pass after the helper generalisation (no regression in adjacent table-table or quote-table cases — those collapse against the previous after-spacer, which still has After > 0).
• New: AddQuote_AfterHeading_ShouldCollapseAgainstHeadingSpaceAfter — heading SpaceAfter=280 + quote SpaceBefore=160 → no spacer between, heading After stays 280.
• New: AddQuote_AfterHeading_ShouldGrowHeadingSpaceAfterWhenQuoteWantsLargerGap — heading SpaceAfter=100 + quote SpaceBefore=320 → heading After grows to 320, no spacer between.
• dotnet test 325/325 passing, 0 errors, 0 new warnings.

Related

• Quote: PaddingSpace cannot express left-side padding #72 (introduced table-cell rendering for quotes)
• Quote (table-rendered): adjacent ::: blocks produce 2 spacer paragraphs and excessive vertical whitespace #76 / fix: collapse adjacent quote-table spacers (#76) #77 (initial adjacent-table collapse — partial fix)

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.07%. Comparing base (19fcd3e) to head (47f194c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...kdownToDocx.Core/OpenXml/OpenXmlDocumentBuilder.cs	77.77%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
+ Coverage   88.90%   89.07%   +0.16%     
==========================================
  Files          28       28              
  Lines        1623     1620       -3     
  Branches      215      213       -2     
==========================================
  Hits         1443     1443              
  Misses        128      128              
+ Partials       52       49       -3     

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