Document the XLSX support boundary

[kiki830621]

Refs #144

Summary

• record the single observed high-value Excel delivery without inferring an unsupported usage frequency
• choose read/write support through feature: che-excel-mcp — 沉澱 Excel 自動化實戰(xlsx/xlsm 產生、VBA 注入、真 Excel 驗證)成 MCP server #135 while keeping macdoc convert free of XLSX routes for now
• document the native macOS mapping and the hostile-OPC, macro-consent, atomic-save, and protected-package boundaries
• pin the genoffice comparison and define concrete reconsideration triggers

Verification

• multi-lens IDD verification: PASS, 0 blocking and 0 follow-up findings
• issue Expected: 4/4; authoritative completion checklist: 5/5
• git diff --check c67b8d8..adcbf89
• docs-only change; no Swift runtime tests required
• Codex cross-model channel was attempted twice but unavailable because of a network failure; this process gap is disclosed in the issue verification report

Verification report: #144 (comment)

Checklist

• diagnosis and implementation plan recorded
• evidence and route counts reproduced from pinned sources
• security and data-integrity boundaries documented
• issue metadata synchronized
• verify-gated PASS

Related work

• feature: che-excel-mcp — 沉澱 Excel 自動化實戰(xlsx/xlsm 產生、VBA 注入、真 Excel 驗證)成 MCP server #135 owns the Excel MCP product boundary
• docs: OPC zip 手術論述 — .xlsm 巨集注入、vbaProject.bin 工作流與 byte 級保真驗證 #136 owns OPC and VBA preservation guidance
• docs: 平台標示慣例 — docs 與 issues 需明示適用平台(macOS/Windows/Linux/跨平台)與實測範圍 #139 owns platform-specific follow-up
• 研究 genoffice docx-engine 的 byte-preserving round-trip（patch-narrowly），對照 ooxml-swift op log #143 is the parent comparison context

Issue 144 remains open. After review and merge, use the IDD closing workflow for issue 144.