OD-core unit tests

Per-crate, no I/O beyond synthetic inputs, parallel-safe. Live under crates/fieldbus-od-core/tests/.

Test Case: Identity, DictEntry, PdoEntry round-trip TEST_0600
status: open
verifies: REQ_0702

Unit tests construct each public type with synthetic values, clone, and compare for structural equality. No I/O. Confirms the public surface compiles and supports the trait derives expected by OD type surface (REQ_0702).
Test Case: fieldbus-od-core has no transport deps TEST_0601
status: open
verifies: REQ_0700

CI shell check: cargo tree -p fieldbus-od-core --no-default-features shall not list ethercrab, socketcan, taktora-connector-can, taktora-connector-ethercat, or any I/O-bearing crate in the resolved graph. Fails the CI job on any match.
Test Case: fieldbus-od-core compiles under no_std + alloc TEST_0602
status: open
verifies: REQ_0701

Compile-only test: a small bin target inside crates/fieldbus-od-core/tests/no_std/ declares #![no_std], uses alloc::vec::Vec, constructs each type. Compiles with --no-default-features; the test passes if compilation succeeds.
Test Case: ethercat-esi re-exports lifted types TEST_0603
status: open
verifies: REQ_0703, REQ_0704

Compile-only test: a test crate under crates/ethercat-esi/tests/reexport/ writes use ethercat_esi::{Identity, DictEntry, PdoEntry, PdoMap}; and constructs each. Compiles if and only if the re-export façade is in place.