Add internal *ValueType backing fields to the IDL

The internal `count` (RTLCV2a) and `entries` (RTLMV2a) properties
were already specified in prose but missing from the IDL block.
Add them, matching the private `_count` and `_entries` fields on
ably-js's `LiveCounterValueType` and `LiveMapValueType`.

Lifted from Sachin's spec-alignment PR [1].

[1] #480

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 2 people

specifications/objects-features.md

@@ -1111,9 +1111,11 @@ Types and their properties/methods are public and exposed to users by default. A
       update: Dict<String, 'updated' | 'removed'> // RTLM18b
 
     class LiveCounterValueType: // RTLCV*
+      count: Number // RTLCV2a, internal
       static create(Number initialCount?) -> LiveCounterValueType // RTLCV3
 
     class LiveMapValueType: // RTLMV*
+      entries: Dict<String, (Boolean | Binary | Number | String | JsonArray | JsonObject | LiveCounterValueType | LiveMapValueType)>? // RTLMV2a, internal
       static create(Dict<String, Boolean | Binary | Number | String | JsonArray | JsonObject | LiveCounterValueType | LiveMapValueType> entries?) -> LiveMapValueType // RTLMV3
 
     interface PathObjectSubscriptionEvent: // RTPO19d