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.

custom_components/localthings/registry/by_type/cooktop.py

@@ -1,11 +1,21 @@
-"""Cooktop device registry."""
+"""Gas cooktop device registry (NA9300K-class, PR #23).
+
+Named 'gas_cooktop' (not 'cooktop') so its diagnostics/device-info label
+doesn't collide with the unrelated induction_cooktop family (issue #86,
+by_type/induction_cooktop.py) -- two different OCF surfaces that happen to
+share the English word "cooktop". The `_REGISTRY_BY_KEY['cooktop']` lookup
+key is unchanged: it's relied on by real devices reporting
+oneUiVersion "Cooktop" (for_device), the legacy ARTIK051 modelNum rule
+(for_device_by_model), and the resource-signature fallback
+(for_device_by_resources) alike.
+"""
 
 from ..capabilities import common, cooktop, ignored
 from ._base import DeviceRegistry, _build
 
 
 REGISTRY = DeviceRegistry(
-    name='cooktop',
+    name='gas_cooktop',
     capabilities=_build([
         *ignored.IGNORED,
         cooktop.COOKTOP_POWER,