DBC frontend tests

Per-crate, no I/O beyond synthetic .dbc text, parallel-safe. Live under crates/taktora-idl-dbc/tests/.

Test Case: DBC parse and lower round-trip TEST_0929
status: implemented
verifies: REQ_0952, REQ_0953
links incoming: REQ_0952, REQ_0953

crates/taktora-idl-dbc/tests/roundtrip.rs: parses fixture DBC text and asserts the parsed structure (version, nodes, messages, signals, multiplexer roles, value tables), then lowers it and asserts the IR (messages → bounded structs, scalar type inference from bit widths, value tables → enums) and the DbcLayout sidecar (per-signal start bit / bit length / byte order, per-frame CAN id and extended-id flag). Confirms parse (DBC parse to a typed model (REQ_0952)) and lower (DBC lower to IR plus layout... (REQ_0953)).