fix(codegen): path-specific drops for struct return from nested scopes

[slepp]

Supersedes #276 (rebased onto current main after 6 major merges).

Problem

return StructType{...} from inside nested control flow (if/for/while) returned the trailing expression value instead of the early return value, because initReturnFlagAndSlot excluded aggregate types from returnSlot creation. Additionally, non-returned variables were never dropped — a memory leak.

Solution

Lazy returnSlot creation with path-specific drop semantics:

• ensureReturnSlot(): creates return slot lazily for aggregate types at the function entry block
• earlyReturnFlag: distinguishes explicit return statements from trailing expressions for drop path selection
• Pre-scan with scanReturns/hasNestedReturn: detects nested returns before body generation to enable return guards
• Path-specific drops: scf.if(earlyReturnFlag) with separate exclusion sets per branch
• Struct alloca promotion: extends declareVariable promotion to struct types (gated by returnSlotIsLazy) with zeroinitializer
• Struct null-guard: checks first pointer field before calling user-defined Drop on potentially uninitialized structs
• FunctionGenerationScope: updated to save/restore new state fields

Tests

3 new E2E tests covering struct return from if-body, for-loop, and outer-scope variable return.

Closes #276