fix: collapse adjacent quote-table spacers (#76)

[forest6511]

Summary

• Closes Quote (table-rendered): adjacent ::: blocks produce 2 spacer paragraphs and excessive vertical whitespace #76 — adjacent ::: note blocks rendered as single-cell tables (since bcecaf5) accumulated two spacer paragraphs between every pair, producing ~2 lines of visible whitespace.
• AddCollapsingBeforeSpacer now collapses each block's before-spacer against an immediately preceding empty spacer paragraph, keeping max(prev.SpaceAfter, next.SpaceBefore) (CSS margin-collapse semantics).
• Applied to both AddQuoteAsTable and AddTable, so the same regression in the rare adjacent-tables case is also fixed.
• After-spacers are still emitted unconditionally — collapse happens at the next block's before-spacer, so the very last table/quote-table still gets its trailing gap.

Approach

Issue #76 listed three options. This PR is Option B (margin-collapse): more correct than the minimal Option A and a tiny amount more code (one helper + one predicate). When two adjacent quote-tables share the same SpaceBefore/SpaceAfter, the collapsed gap equals one spacer paragraph — matching the bcecaf5-prior paragraph rendering.

The detection (IsBodySpacerParagraph) treats any empty body-level paragraph with Before="0" and a non-empty After as collapsible. Heading after-spacers match this shape too, which is intentional — collapsing against a heading's after-spacer with max() never shrinks the gap, only avoids stacking.

Test plan

• dotnet test (323 / 323 passing, 0 warnings introduced)
• New: AddQuote_AdjacentTableQuotes_ShouldCollapseSpacersToSingleParagraph — three back-to-back quote-tables, asserts exactly one body element between every pair.
• New: AddQuote_AdjacentTableQuotes_ShouldKeepLargerCollapsedSpacing — differing SpaceBefore/SpaceAfter collapse to max().
• New: AddTable_AdjacentTables_ShouldCollapseSpacersToSingleParagraph — same rule for plain Markdown tables.
• (recommended) Re-run a Vol.2 build of forest6511/rust-textbook-advanced and remove the Python post-processing workaround — no longer needed.

[codecov]

Codecov Report

❌ Patch coverage is 64.70588% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.90%. Comparing base (ee63cf7) to head (447d2ca).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...kdownToDocx.Core/OpenXml/OpenXmlDocumentBuilder.cs	64.70%	0 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #77      +/-   ##
==========================================
- Coverage   89.17%   88.90%   -0.27%     
==========================================
  Files          28       28              
  Lines        1608     1623      +15     
  Branches      206      215       +9     
==========================================
+ Hits         1434     1443       +9     
  Misses        128      128              
- Partials       46       52       +6     

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