CAN/DBC backend

The one concrete backend this round (taktora-idl-codegen-can (BB_0115)): given the DBC layout sidecar, it emits a WireType implementation per message that bit-packs each field at the start bit, length, and byte order the DBC declared, calling only the taktora-idl-wire primitives. A build-time verification harness (taktora-idl-codegen-can-tests (BB_0116)) proves the generated code compiles and round-trips.

Feature: CAN/DBC backend FEAT_0115
status: implemented
satisfies: FEAT_0110
is satisfied by: REQ_0956, REQ_0957
is implemented by: BB_0115, BB_0116

A new crate taktora-idl-codegen-can implementing MessageBackend for CAN frames, plus the taktora-idl-codegen-can-tests verification harness that exercises the full .dbc → IR → codegen → generated-code pipeline.
Requirement: CAN backend emits WireType impls via the wire runtime REQ_0956
status: implemented
satisfies: FEAT_0115
is verified by: TEST_0932, TEST_0933, TEST_0931
links outgoing: BB_0115, TEST_0931, TEST_0932, TEST_0933
links incoming: RISK_0025

taktora-idl-codegen-can shall implement MessageBackend so that, given a DbcLayout sidecar, it emits one WireType implementation per message struct whose encode/decode bit-pack each field at the DBC-declared start bit, length, and byte order, calling only the taktora-idl-wire primitives (CAN signal bit-packing prim... (REQ_0951)) and never serde (no_std, allocation-free, se... (REQ_0950)). Naming and field classification arrive already resolved from MessageBackend trait and re... (REQ_0955); the backend adds only the wire format. Integer and enum fields are emitted this round; bool and float fields are rejected (see Anti-goals and cross-cutting traceability).
Requirement: Build-time generation proof-of-life REQ_0957
status: implemented
satisfies: FEAT_0115
is verified by: TEST_0932
links outgoing: BB_0116, TEST_0932

The taktora-idl-codegen-can-tests crate shall generate WireType code from a fixture .dbc in its build.rs — parse, lower, generate, parse the tokens as Rust, format with prettyplease, and write to OUT_DIR — and compile the result into the crate so the round-trip tests exercise real generated code. A failure at any pipeline stage shall fail the build.