fix(odt): continue list numbering across loop copies, process header/footer regions, drop dead {{#empty}} markers

[vaceslav]

Follow-up to #138 (closed), from the independent review of the OpenDocument support.

Problem

1. Numbered lists restart in loops (HIGH). A paragraph-level loop over a numbered item (in LibreOffice a text:list of its own) cloned the list once per iteration. Every clone was a separate list, so LibreOffice rendered 1. A / 1. B / 1. C, while the same DOCX template renders 1. / 2. / 3.. This also affected DOCX templates converted to ODT by LibreOffice (soffice --convert-to odt) and numbered lists in repeated table rows.
2. Header/footer regions ignored (MEDIUM). style:region-left / -center / -right inside style:header / style:footer were neither processed nor validated. Placeholders, conditionals and loops in them were left as they were.
3. Dead {{#empty}} handling (LOW). OdtTemplateEngine and the DOCX DocumentWalker still treated paragraphs containing {{#empty}} / {{/empty}} as marker paragraphs, although the feature was removed in refactor: dedupe marker text and quote normalization, remove dead internal code (#155) #190.

Fix

• OdtTemplateEngine.ExpandLoop marks every clone of a list with the template list it comes from (an in-memory XObject annotation, also through nested loops). After OdtUniqueNames.EnsureUnique, the new ContinueListNumbering step gives the first copy still in the document an xml:id (it keeps the template's id or gets an unused templify-listN). Every later copy gets text:continue-list="<that id>", and any copied continue-numbering or xml:id is removed from it. continue-list (ODF 1.2) links the copies even when other lists come between them. Only copies of the same source list are linked, so a list after the loop still restarts. Sub-lists (lists inside a list, including list-item loops) are not changed. Nested loops continue across outer iterations, like the shared numId in DOCX.
• style:region-* are containers in OdtTemplateEngine.ProcessBlock and in OdtTemplateValidator.GetChildSequences (new OdfNames.IsHeaderFooterRegion).
• Removed the {{#empty}} / {{/empty}} checks in both engines. Behavior change: a paragraph that contains the literal text {{#empty}} now gets placeholder replacement like any other paragraph. Before, its placeholders were skipped silently.
• docs/for-template-authors/libreoffice.md: list numbering continues across loop iterations, and header/footer regions are supported.

LibreOffice results (headless LibreOffice 26.8, txt export). Paragraph loop with a numbered list, a second list and text between them: 1. A / 2. B / 3. C, and the second list separately 1. / 2. / 3.. A list after the loop: 1.. Nested loops: 1. 2. | 3. 4.. Table-row loop with a numbered list in a cell: 1. 2. 3.. For a DOCX template converted by LibreOffice, the ODT output now matches the DOCX output line for line, including nested loops. Note: LibreOffice Writer drops header regions on load, so for regions the round trip only checks that the processed document opens and converts.

Tests

• OdtLoopTests: 8 new tests covering continuation, generated ids, unrelated and single-iteration lists left alone, a clone removed by a conditional, nested loops, table-row loops, nested sub-lists and list-item loops.
• OdtContainerTests.HeaderAndFooterRegions_AreProcessed covers placeholders, a conditional and a loop in regions. OdtValidationTests.HeaderRegions_AreValidated covers a syntax error and loop-scope missing variables in regions.
• OdtPlaceholderTests / DocumentTemplateProcessorEdgeCaseTests: a paragraph with {{#empty}} text is ordinary text (ODT and DOCX).
• LibreOffice round trips (Category=LibreOffice, run locally): NumberedListsInLoops_ContinueTheirNumberingInLibreOffice, DocxConvertedByLibreOffice_ProcessedAsOdt_MatchesDocxProcessing (ported from the review probe, now expects 1/2/3) and HeaderRegions_ProcessedDocumentOpensInLibreOffice.
• All 14 new tests fail without the fix. The full suite passes on net10/9/8 with the LibreOffice tests included, and so do the Release CI build, dotnet format --verify-no-changes, dotnet pack and mkdocs build --strict.

Public API impact: none. There is one behavior change for the edge case of literal {{#empty}} text, described above.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 96.61017% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...riasDev.Templify/OpenDocument/OdtTemplateEngine.cs	98.07%	0 Missing and 1 partial ⚠️
TriasDev.Templify/Visitors/DocumentWalker.cs	0.00%	0 Missing and 1 partial ⚠️

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