BCM path proven on X520: fix multi-byte i2c read (SDA release), GET_SNR is 0x8030 invoked bare, IEEE SNR regs dead on BCM, module pause forced off

docs/goals.md

@@ -6,7 +6,7 @@ The target of measurement is the cable, not throughput. Line rate is a means to
 
 1. **Loss and error attribution** — reception gaps, link errors, NIC/driver counters as first-class output alongside application loss. Baseline loss must be exactly zero before a run counts; any host-side loss masks real cable faults.
 2. **Noise tolerance** — a deliberately-bad "noise" cable intertwined with the test cable, driven by link up/down cycling, stresses the cable under test with alien crosstalk.
-3. **Per-pair SNR margin** from the module PHYs (IEEE 802.3an standard registers) — the leading indicator of a marginal cable before it drops frames.
+3. **Per-pair SNR** from the module PHYs — the leading indicator of a marginal cable before it drops frames. IEEE 802.3an standard registers on the Marvells; the BCM leaves those unpopulated and reports through its vendor command handler instead (transports.md).
 4. **Cable length** — sanity check and fault localization. Sources, in preference order:
    - Module PHY DSP estimate — works on a linked cable.
    - PHY TDR — localizes opens/shorts both-ended; healthy-cable length only single-ended.