fix(session): support formatted insertion in leading-tab runs

[JSv4]

Styled typing inside a run beginning with a tab returned offset_out_of_range, including the NVCA footnote at native offset 24. Allow the existing splitter to accept leading tabs followed only by text, keeping tabs before the retained prefix and applying typing formatting only to the inserted text.

The change retains the lightweight transaction path, failure rollback, and single-step undo/redo. Fields, inline containers, surrogate-pair boundaries, and other mixed run content remain guarded. Regression coverage extends the existing theories and exercises the exact NVCA footnote through both .NET and the public npm API. Documentation and changelog are updated.

Validation:

• Five new insertion/rollback cases failed before the fix.
• 1,267 DocxSession tests passed (dotnet test Docxodus.Tests/Docxodus.Tests.csproj --filter FullyQualifiedName~DocxSession).
• All 8 Chromium tests in npm/tests/atomic-batch.spec.ts passed against a fresh full WASM build.
• npm run build, npm run typecheck, and git diff --check passed.
• GPT-6 Astra adversarial review completed with no remaining findings.

Closes #802.