Codegen tests

Unit tests of the plane-generic codegen and the CAN backend’s token emission. No build-time codegen here — the generated-code round-trips live on CAN backend / generated-code tests.

Test Case: Naming policy and identifier-collision detection TEST_0930
status: implemented
verifies: REQ_0954
links incoming: REQ_0954, REQ_0955

Inline #[cfg(test)] cases in crates/taktora-idl-codegen/src/lib.rs and crates/taktora-idl-codegen/src/naming.rs: assert type / field / variant naming (PascalCase, snake_case), Rust-keyword escaping, and leading-digit prefixing, and assert that two distinct source names normalising to one identifier (type, field, or variant) is rejected as a collision rather than silently emitted. Confirms Deterministic naming policy... (REQ_0954).
Test Case: CAN backend emits parseable WireType tokens TEST_0931
status: implemented
verifies: REQ_0955, REQ_0956
links incoming: REQ_0956, REQ_0955

Inline #[cfg(test)] case in crates/taktora-idl-codegen-can/src/lib.rs: parses a sample DBC, lowers it to IR plus layout, drives generate with CanBackend, and asserts the emitted TokenStream parses as valid Rust and contains the expected elements (a WireType impl, the value-table enum, snake_case fields, and pack_signed calls). Confirms the MessageBackend / generate contract (MessageBackend trait and re... (REQ_0955)) and the CAN emission (CAN backend emits WireType ... (REQ_0956)).