Runtime trait surface tests

Live under crates/canopen-eds-rt/tests/.

Test Case: CanOpenDevice trait shape compiles for a hand-written device TEST_0640
status: open
verifies: REQ_0750, REQ_0754, REQ_0755

Hand-written test impl of CanOpenDevice for a minimal MockDevice validates the trait surface compiles end-to-end. Asserts IDENTITY is reachable, node_id / set_node_id round-trip, nmt_state / set_nmt_state round-trip, on_rpdo is callable on &mut self with idx: u8 in 0..=3, drain_tpdos is callable with a TpdoSink, and each error path returns the expected CanOpenError variant (PdoLenMismatch, NmtStateViolation, AbortCode, TransportFailed).
Test Case: CanOpenConfigurable async trait shape compiles TEST_0641
status: open
verifies: REQ_0751

Compile-only test: a mock device implements CanOpenConfigurable and an async fn configure body driving a mock SdoClient. The test passes if compilation succeeds; catches any trait-method-async surface drift.
Test Case: canopen-eds-rt is the trait home, not taktora-internal TEST_0642
status: open
verifies: REQ_0752

CI shell check: rg "trait CanOpenDevice" across the workspace matches exactly one source location, inside crates/canopen-eds-rt/src/. Same check for trait CanOpenConfigurable.
Test Case: PdoOut payload uses heapless::Vec<u8, 8> TEST_0643
status: open
verifies: REQ_0753

Compile-time check: static_assertions::assert_type_eq_all! on PdoOut::payload’s type vs heapless::Vec<u8, 8>. A change to the buffer type (e.g. lifting to a const generic for CAN-FD) requires this test to be updated deliberately rather than passing accidentally.
Test Case: RPDO rejected outside Operational state TEST_0644
status: open
verifies: REQ_0756

Integration test against a generated device fixture: set nmt_state(NmtState::PreOperational), call on_rpdo with a valid frame, assert the returned error is CanOpenError::NmtStateViolation. Switch to NmtState::Operational and confirm the same frame succeeds.