Add stream-contract conformance checkers

[timkpaine]

A reusable checker for the valid/ready stream contract: valid held and payload stable until ready fires, transfers only when both are high, exactly one last per batch, no transfer after last, and status held until status_ready. Each violation raises naming the RULE and the CYCLE.

Motive: real bugs shipped past hand-written benches in the last few days — a core whose output was combinational off a register that had already advanced (right record counts, wrong contents), a core sustaining 1.25 instead of 1.0 records per cycle, a bench where deleting an entire error branch still passed, and one where hardwiring output_last low still passed. These properties get hand-written per bench today, inconsistently.

Ten tests drive deliberately BROKEN modules — dropping valid, mutating a stalled payload, emitting two lasts, never emitting last — and assert each fails with its rule identified. That is the proof the layer is worth having rather than a claim about it.

The useful conclusion is about tooling, not code. This was scoped as an evaluation of dau-sim's CSP simulator as a second simulation path alongside Verilator. It turns out not to need one: the checker is simulator-NEUTRAL Python that samples signal values, so it attaches to the existing Verilator/cocotb benches directly. The value was never a second simulator — it was that these properties should be enforced generically instead of re-derived per bench. Adopting it costs no new simulation path and no second harness.

Additive only; the existing cocotb integration's behaviour and defaults are untouched. Suite: 10 passed.