fix(visitor-plugin-common): keep deferred fields optional with overlapping named @defer spreads

[redox]

Summary

• Named @defer/@stream fragment spreads were typed as a single union of all fragment fields. When a field like id was also selected outside the spread, intersecting with the required outer field eliminated the absent branch and made deferred-only fields required.
• Expand incremental named spreads into per-selection nodes (same shape as inline @defer) so each deferred field stays its own optional union. Fragment masking (inlineFragmentTypes: "mask") is unchanged and still uses Incremental<> fragment refs.

Test plan

• Added regression test for overlapping id + named @defer (fails before fix, passes after)
• Updated related @defer / @skip+@defer snapshots for per-field unions
• vitest incremental delivery + skip/include defer suites green
• Client-preset @defer tests green

Made with Cursor

[redox]

Opened upstream instead: will link after create

[redox]

Opened upstream instead: dotansimha#10898