"""Combined-rate comprehension organ (off-serving) — two explicit rates over one shared unit.

The semantic object R3 (single-rate) does NOT have: an **effective rate** formed by combining two
explicit rates that share a compound unit, under an explicit ``combine_mode``:

```text
effective_rate = rate_a + rate_b   (sum        — cooperation: "working together")
effective_rate = rate_a - rate_b   (difference — opposing flow: "fills ... drains")
```

then the single-rate algebra over that effective rate (``quantity = effective_rate × time``).

A separate organ, deliberately NOT folded into ``rate_comprehension``: combined rate is not
single-rate with a bigger parser — it introduces a new model and ruler. If the rate organs later
converge, a shared rate algebra can be extracted then. Off-serving: imports no
``generate.derivation`` / ``core.reliability_gate``, so it cannot move the sealed GSM8K serving
metric. Deterministic; no clock, no randomness.

CMB-a (this slice) defines the model + the independent gold ruler + the setup oracle only. It
claims exactly one thing: *the combined-rate setup ruler is defined and gold-valid.* No reader
(CMB-c), no solver (CMB-b), no router/contemplation wiring (CMB-d) — and therefore no capability
claim yet.
"""
