CAN backend / generated-code tests

End-to-end tests that compile real generated WireType code. The crates/taktora-idl-codegen-can-tests harness (publish = false) runs the .dbc → IR → codegen → format pipeline in its build.rs, compiles the generated module, and round-trips it. These tests double as the verification of the taktora-idl-wire primitives, which have no unit tests of their own.

Test Case: Generated round-trip, DLC sizing, little-endian placement TEST_0932
status: implemented
verifies: REQ_0949, REQ_0950, REQ_0951, REQ_0956, REQ_0957
links incoming: REQ_0956, REQ_0957, REQ_0949, REQ_0950, REQ_0951

crates/taktora-idl-codegen-can-tests/src/lib.rs: with the module generated at build time, asserts MAX_SERIALIZED_LEN equals the frame DLC, that an encode/decode cycle preserves field values, and that a 16-bit little-endian signal packs LSB-first at the declared bit offset (golden byte placement). Confirms the WireType surface (WireType trait surface (REQ_0949)), the allocation-free path (no_std, allocation-free, se... (REQ_0950)), the little-endian primitive (CAN signal bit-packing prim... (REQ_0951)), the CAN emission (CAN backend emits WireType ... (REQ_0956)), and the build-time generation pipeline (Build-time generation proof... (REQ_0957)).
Test Case: Enum and signed-enum round-trip; unknown value rejected TEST_0933
status: implemented
verifies: REQ_0951, REQ_0956
links incoming: REQ_0956, REQ_0951

Asserts a value-table enum field encodes and decodes, that an out-of-range discriminant decodes to WireError::UnknownEnumValue, and that a signed enum variant (e.g. -1 on a 4-bit signed signal) round-trips through two’s-complement encode and sign-extending decode. Confirms signed/unsigned primitives (CAN signal bit-packing prim... (REQ_0951)) and the backend’s enum emission (CAN backend emits WireType ... (REQ_0956)).
Test Case: Wire error path — buffer and value bounds rejected TEST_0934
status: implemented
verifies: REQ_0949, REQ_0951
links incoming: REQ_0949, REQ_0951

Asserts that encode/decode against an undersized buffer returns WireError::BufferTooSmall, and that a field value that does not fit its declared bit width returns WireError::ValueOutOfRange. Confirms the closed deterministic error set of the WireType surface (WireType trait surface (REQ_0949)) and the primitives (CAN signal bit-packing prim... (REQ_0951)).
Test Case: Multiplexed signals flatten into one struct TEST_0935
status: implemented
verifies: REQ_0953
links incoming: REQ_0953

Asserts that a multiplexor plus a multiplexed signal in the fixture DBC lower into plain fields of one generated struct (the slice-1 flattening behaviour) and round-trip. Confirms the lowering rule in DBC lower to IR plus layout... (REQ_0953).