Rename gas cooktop registry's display name to avoid induction_cooktop confusion

Renames DeviceRegistry.name from 'cooktop' to 'gas_cooktop' for the
NA9300K-class gas-cooktop registry (PR #23), so diagnostics/device-info
labels no longer collide with the unrelated induction_cooktop family
(issue #86) -- two different OCF surfaces that happen to share the
English word "cooktop".

Safe rename: _REGISTRY_BY_KEY's 'cooktop' lookup key is unchanged, so
all three existing detection paths (oneUiVersion "Cooktop" exact
match, the legacy ARTIK051 modelNum rule, and the resource-signature
fallback) keep routing real devices exactly as before. Entity
unique_ids are built from device serial + entity key, not registry
name, so existing entities are unaffected. Only the DeviceInfo.name
and diagnostics device_type strings change, both cosmetic.

README.md

@@ -25,7 +25,7 @@ Your state stays on your LAN: HA talks to the appliance over a direct DTLS sessi
 | Air purifier | `by_type/air_purifier.py` |
 | Dryer | `by_type/dryer.py` |
 | Oven | `by_type/oven.py` |
-| Cooktop (read-only burner status) | `by_type/cooktop.py` |
+| Gas cooktop (read-only burner status) | `by_type/cooktop.py` |
 | Range hood | `by_type/range_hood.py` |
 | Range | `by_type/range.py` |
 | Dishwasher | `by_type/dishwasher.py` |