Add minimal XLSX export support

[donatj]

Summary

• add a minimal XlsxEngine that mirrors SpreadsheetML cell support
• package worksheets as streamed Office Open XML using the existing ZipStream dependency
• add integration coverage and CI ZIP support

Validation

• vendor/bin/phpunit
• vendor/bin/phpcs
• vendor/bin/php-cs-fixer fix --dry-run --diff --sequential
• LibreOffice headless XLSX-to-CSV smoke test

[Copilot]

Pull request overview

Adds first-class XLSX export capability to the exporter by introducing a new XlsxEngine that writes a minimal Office Open XML workbook as a streamed ZIP archive, plus integration coverage and CI/dev dependencies to support ZIP-based validation.

Changes:

• Introduce XlsxEngine to generate a minimal .xlsx (OOXML) archive via ZipStream.
• Add an integration test that validates the produced ZIP entries and key XML values (workbook, sheets, core properties).
• Update docs and CI/dev requirements to include XLSX support and ZIP tooling.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/Integration/XlsxTest.php	New integration test that validates XLSX ZIP contents and XML structure/values.
src/Engines/XlsxEngine.php	New streaming OOXML XLSX writer using ZipStream, with minimal worksheet/style/core-props support.
README.md	Document XLSX as a supported format and add XlsxEngine API docs.
composer.json	Add ext-zip to dev requirements (used by integration test validation).
.mddoc.xml.dist	Update supported formats list to include XLSX.
.github/workflows/ci.yml	Ensure CI installs the zip extension to run XLSX-related tests.

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[Copilot]

Copilot review overview

🟡 Changes recommended

XLSX relationship metadata, sheet-name validation, and repeated-export state require correction.

Review effort: Balanced
Findings: 1

Open (1)

• Validate worksheet names against XLSX constraints · New

Resolved since last review (2)

• processSheet() stores per-worksheet php://temp streams in $worksheetData, but…
• corePropertiesXml() uses $this->createdTime ?: time(), which treats 0 as falsy and will…

[Copilot]

Copilot review overview

🟡 Changes recommended

The relationship metadata URI is incorrect, and accepted control characters can produce malformed workbook XML.

Review effort: Balanced
Findings: 1

Open (1)

• Reject XML-forbidden characters before creating worksheet text nodes · New

Resolved since last review (1)

• Validate worksheet names against XLSX constraints

Previously missed (1)

In code that hasn't changed since last review

Use OPC namespace for core-properties relationship

src/​Engines/​XlsxEngine.php:292

The core-properties relationship type uses the office-document namespace, but this relationship is defined by OPC under http://schemas.openxmlformats.org/package/2006/relationships/metadata/core-properties. Package-aware consumers may therefore fail to discover docProps/core.xml; use the package relationships namespace here.