fix(lowering): eliminate spurious E3002 alongside E3001 for moved struct members - #9994

Summary

When a non-copy variable is moved (used more than once), the borrow checker previously created a fresh dummy variable to represent the already-moved value. This dummy variable had no corresponding lowering statement, which caused spurious E3002 ("Variable not dropped") errors to appear alongside the real E3001 ("Variable was previously moved") error. Additionally, when assembling scattered (destructured) struct values that contained a moved member, the reconstructed struct variable was discarded and a new dummy variable was returned, causing the struct reconstruction statements to be emitted but their result to be orphaned.

The fix changes MovedVar to carry the original VariableId instead of just the TypeId. When a move error is detected, the original variable ID is reused rather than allocating a new dummy. When assembling a scattered struct that contains a moved member, the struct is still reconstructed (so the statements are valid), and the resulting variable ID is propagated through the MovedVar error. A new test case (Spurious E3002 alongside E3001 for orphaned inner struct construction) is added to confirm the fix.

Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

Why is this change needed?

When a non-copy variable was used after being moved, the compiler emitted a spurious E3002 error in addition to the correct E3001 error. This happened because a dummy variable was created to stand in for the moved value, but no statement ever defined that dummy variable, leaving it "undropped" from the borrow checker's perspective. Similarly, struct reconstruction statements were being emitted for orphaned variables, producing inconsistent lowering IR.

What was the behavior or documentation before?

• A use-after-move of a non-copy variable produced both E3001 (variable was previously moved) and a spurious E3002 (variable not dropped) diagnostic.
• The E3002 note incorrectly pointed to the second use site and cited a potential panic as the reason, rather than pointing to the divergence caused by the if branch.
• Scattered struct assembly discarded the reconstructed variable when a member was moved, leaving orphaned struct_construct statements in the lowered IR.

What is the behavior or documentation after?

• A use-after-move of a non-copy variable produces only the correct E3001 diagnostic.
• The E3002 note now correctly identifies the divergence (e.g., an if block) rather than a panic site.
• Scattered struct assembly always emits and uses the reconstructed variable, keeping the lowered IR consistent.

Related issue or discussion (if any)

Bug repro captured in the new test: Spurious E3002 alongside E3001 for orphaned inner struct construction.

Additional context

The PanicDestruct trait note was also removed from the E3002 diagnostic output, as it is no longer emitted in the updated test expectations.