Fix #7996 mutation viewer selection across subgraphs

[michaelstaib]

• add a Fusion ASP.NET Core regression test for mutation payload viewer selecting fields from another subgraph
• allow viewer shared-path fallback from non-query operations when Query.viewer exists on the target schema
• emit query operations for query-root fallback work items and map the fallback fetch via source: $.viewer and target: $.doSomething.viewer
• preserve merge paths for non-root operations without requirements by creating a scoped variable path for result integration

Fixes #7996

[github-actions]

Fusion Gateway Performance Results

Progress: 0/6 benchmarks completed — this report updates as each job finishes.

Simple Composite Query

	Req/s	Err%
Constant 1 (50 VUs)	pending	pending
Ramping 1 (0-500-0 VUs)	pending	pending

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 1	pending	pending	pending	pending	pending	pending
Ramping 1	pending	pending	pending	pending	pending	pending

---

Deep Recursion Query

	Req/s	Err%
Constant 1 (50 VUs)	pending	pending
Ramping 1 (0-500-0 VUs)	pending	pending

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 1	pending	pending	pending	pending	pending	pending
Ramping 1	pending	pending	pending	pending	pending	pending

---

Variable Batching Throughput

	Req/s	Err%
Constant 1 (50 VUs)	pending	pending
Ramping 1 (0-500-0 VUs)	pending	pending

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 1	pending	pending	pending	pending	pending	pending
Ramping 1	pending	pending	pending	pending	pending	pending

---

Runner 1 = benchmarking-1

Run 22761052196 • Commit b2e55f2 • Fri, 06 Mar 2026 11:15:18 GMT

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (afd4ba4) to head (38be732).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff      @@
##   main   #9271   +/-   ##
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