Runtime trait surface tests

Live under crates/ethercat-esi-rt/tests/.

Test Case: EsiDevice trait shape compiles for a hand-written device TEST_0430
status: open
verifies: REQ_0530

Hand-written test impl of EsiDevice for a minimal MockDevice validates the trait surface compiles end-to-end. Asserts IDENTITY, input_len, output_len return the expected values, and that decode_inputs / encode_outputs round-trip a synthetic BitSlice<u8, Lsb0>.
Test Case: EsiConfigurable async trait shape compiles TEST_0431
status: open
verifies: REQ_0531

Compile-only test: a mock device implements EsiConfigurable with type Assignment = MockAssignment and an async fn configure body. The test passes if compilation succeeds; the async signature shape catches the trait-method-async constraint.
Test Case: ethercat-esi-rt is the trait home, not taktora-internal TEST_0432
status: open
verifies: REQ_0532

CI shell check: rg "trait EsiDevice" across the workspace matches exactly one source location, inside crates/ethercat-esi-rt/src/. Same check for trait EsiConfigurable.