Fuse tuple field assignments - #1299

Summary

• apply scalarized tuple component selection to lvalues as well as reads
• emit only the selected backing store for direct, nested, member, and read-modify-write assignments
• retain one-time index evaluation and index-before-RHS ordering
• leave whole-tuple assignments on the existing all-component path

Verification

• ./gradlew test --tests "tests.wurstscript.tests.LuaBackendAuditTests.tupleFieldAssignmentsOnlyTouchTheSelectedStorageComponent" --tests "tests.wurstscript.tests.LuaBackendAuditTests.tupleFieldReadsOnlyLoadTheSelectedStorageComponent" --tests "tests.wurstscript.tests.LuaBackendAuditTests.tupleMemberArrayAssignmentCapturesIndex" --tests "tests.wurstscript.tests.LuaBackendAuditTests.tupleAssignmentCapturesLvalueBeforeRhs"
• ./gradlew test --tests "tests.wurstscript.tests.TupleTests"
• the new regression failed on the parent commit with discard calls for every unselected backing field, then passed with only the selected stores

Scope

• no inliner, register-budget, spill fallback, assurance, or arithmetic lowering changes
• production-map discard counts, spill counts, and grill test remain to be verified in the map repository