Design: mark only the changed words in a tracked changeText (#28)

[ilia-sokolov]

This thread is for the design questions behind #28: an opt-in way for a tracked changeText to mark only the words that actually change.

Why

In the round-3 agent trial, the same edits came out very differently in Word:

Edit	Hand-written Open XML SDK code	OfficeAgent today
"thirty days" to "forty-five days"	10 of 12 marked only thirty forty-five	Strikes whatever was anchored, usually "thirty days"
Add "125% of" to the liability cap	3 of 3 wrote a pure insertion of "125% of"	2 of 2 struck the whole 60-character clause and retyped it

A reviewer sees a whole clause deleted and rewritten to add two words. No plan can currently express a pure insertion inside a paragraph.

The decided direction (see #28)

• ChangeTextOp.Granularity: Span (today's behaviour, the default in 1.x) or Minimal.
• A connection-level default beside DefaultChangeMode, so a host can switch it on for every agent.
• Under Minimal, trim the common leading and trailing words of expect and with, and write revisions only for the differing middle. An empty middle on either side becomes a pure insertion or a pure deletion.
• Minimal becomes the default in 2.0.

Open questions

1. Word boundaries. Whitespace only, or punctuation too? Is "45" against "forty-five" one word or several?
2. Formatting. What happens when the differing middle crosses runs with different formatting? The proposal is to fall back to Span for that operation. Should that be silent, or reported in the preview?
3. Preview. The preview should show the narrowed span, so a reviewer sees exactly what will be marked. Is a narrowed Before/After enough, or does the change report need a new field?
4. Naming. Granularity with Span and Minimal, or something closer to Word's own vocabulary?
5. Agents. Should the connection default be on for the MCP server's own default configuration in 1.x, or stay opt-in everywhere until 2.0?

Comments, counter-proposals and examples from real review workflows are welcome.