Make mutation batches atomic

[JSv4]

Summary

• add full-package, nested LIFO DocxSession transactions and atomic-by-default mutation batches
• collapse successful atomic batches to one version/undo unit and restore package, history, configuration, revision generators, and relationship topology on failure
• expose consistent atomic/best-effort contracts through JSON/Ops, WASM/npm, Python/stdio, and MCP; retain apply as an explicitly deprecated best-effort alias
• add structured indexed failures, upfront MCP argument/precondition validation, docs, and regression coverage

Verification

• dotnet test ... --filter "DocxSessionAtomicBatchTests|McpServerDispatcherTests|Precondition": 97/97
• targeted atomic/precondition subset: 39/39
• Python atomic/precondition tests: 4/4
• Playwright Chromium atomic batch tests: 2/2
• MCP and Python-host builds, npm typecheck, full WASM/npm build and 4 MiB size budget
• package-byte/custom-XML/relationship rollback checks and git diff --check

Stack

Draft stacked on agent/issue-454-page-citations in the epic's dependency-correct linear chain. #445 still consumes the version/precondition primitive from #447; #454 is placed immediately after #447 so its renderer/page-map work is integrated before the transport-heavy mutation layers. Retarget to main as dependencies land.

Fixes #445