T1b reconciliation and preserve keyed segments

[VolkerChristian]

This pull request adds a detailed investigation document, design/t1-item-upsert-investigation.md, to record findings and recommendations regarding the T1 item-upsert reconciliation issue. No code changes are made to src/ as the investigation concludes that the current structure does not justify implementation changes. The document reviews prior attempts, explains the technical constraints, and outlines the requirements and limitations for any future work on this topic.

Key additions in the investigation document:

Findings and Analysis:

• Explains why the previous implementation attempt (c680e37) failed, specifically in cases where an exact content append and an item upsert were coalesced, resulting in missing widgets due to early returns in the render path.
• Details the limitations of the current reconciliation logic, highlighting that it does not perform an exhaustive keyed diff and can delete unchanged widgets during certain operations.

Implementation Guidance:

• Outlines the essential operations required for a pure item upsert, noting which parts of the current rendering pipeline are necessary and which are not, and why a simple boolean gate is insufficient for a correct solution.
• Provides a call-site audit and runtime proof requirements for any follow-up implementation, ensuring future changes are precisely targeted and verifiable.

Scope and Structural Constraints:

• Clarifies the limitations of the current public API