Fix IL2080 for conditional tuple deconstruction

[Copilot]

Conditional tuple deconstruction lost element-level dataflow across Roslyn flow captures, causing known Type values to be modeled as unannotated ValueTuple.ItemN fields and emit false IL2080 warnings.

Changes

• Track captured tuple elements by capture ID and nested element path.
• Merge branch values through the existing local-state lattice.
• Apply captured values only to compiler-distributed conditional deconstruction; materialized tuples and switch expressions retain field-based analysis.
• Preserve diagnostics for genuine annotation mismatches.

(var methodName, var type) = condition
    ? ("First", typeof(FirstType))
    : ("Second", typeof(SecondType));

_ = type.GetMethod(methodName); // No false IL2080

• Fixes IL2080 from deconstruction assignment "using" ValueTuple #133911

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