flat codegen: data-carrying (tagged-union) enums + value-producing match

Follow-up to hiraditya/Vx.1#227 (which landed payload-free C-like enums: construction + statement-form match now lower through the flat path).

What still declines.

• Data-carrying variants — Option<i32>::Some(42), any enum E { V(T) }. The AST codegen models these as an !llvm.struct<(i32, payload)> (tag + payload), built with llvm.mlir.undef + insertvalue; a match binds the payload via extractvalue (src/codegen/lower/expr.rs::EnumVariantExpr, lower/mod.rs::generate_match_chain). The flat path treats every enum in enum_variants (payload-free only) as a bare i32 and declines the rest.
• Value-producing match — a match in tail/expression position whose arms yield a value (e.g. Option::unwrap's Some => v / None => assert). The flat lower_match is statement-only.
• Payload bindings / non-enum patterns — Pattern::EnumVariant(.., Some(payloads)), Pattern::Literal, Pattern::Identifier.

Concrete corpus case: tests/backend/pass/option_unwrap.vx (Option<i32>::Some + .unwrap() + Vec<Option<i32>>), which also needs generic Option + Vec/stdlib methods.

Approach. Model a data-carrying enum as an aggregate (the flat path already has struct alloca/GEP/field load-store from hiraditya/Vx.1#199/#215): construct via field stores (tag at offset 0, payload after), and extend lower_match to (a) load the tag from the aggregate for the compare and (b) bind a payload pattern via a field load into the arm scope. Add a value-producing match form (each arm stores its result into a slot, like the value-if in hiraditya/Vx.1#201). Overlaps generic-enum monomorphization (Option<T>).

Acceptance. An Option<i32> construct + match/unwrap program lowers through the flat path with JIT parity vs --legacy-codegen; full-corpus parity holds.

Part of the flat-codegen convergence (#200/#201).

Start with src/codegen/lower/expr.rs::EnumVariantExpr and lower/mod.rs::generate_match_chain/lower_match, then inspect the existing flat-path struct alloca/GEP and field load-store support. Use tests/backend/pass/option_unwrap.vx as the concrete case and compare against --legacy-codegen. Done means the Option construction, match/unwrap, and full corpus achieve flat-path JIT parity.