Avoid per-write virtual dispatch in `DictionaryValuesWriter.shouldFallBack()` by caching the size-exceeded check

### Describe the enhancement requested

`DictionaryValuesWriter.shouldFallBack()` is called by `FallbackValuesWriter.checkFallback()` after every single value write. The current implementation dispatches a virtual call to `getDictionarySize()` on every invocation:
```java
public boolean shouldFallBack() {
return dictionaryByteSize > maxDictionaryByteSize || getDictionarySize() > MAX_DICTIONARY_ENTRIES;
}
```
`getDictionarySize()` is an abstract method overridden in each typed subclass (Binary, Long, Double, Integer, Float) to return the backing map's `.size()`. Since `shouldFallBack()` is polled after every write, including writes of duplicate values that do not grow the dictionary, the virtual dispatch and map-size query are redundant work for the common case where most values are already in the dictionary.
Both `dictionaryByteSize` and the dictionary entry count can only increase when a new entry is added (inside the `if (id == -1)` branch of each subclass's write method). Therefore the size-exceeded condition can only transition from `false` to `true` at that exact point.

### Proposal
Replace the per-write check with a cached boolean `dictionarySizeExceeded` flag. Introduce a `checkDictionarySizeLimit(int newDictionarySize)` method that subclass write methods call only when a new dictionary entry is actually added. `shouldFallBack()` then returns the cached flag directly, a simple field read with no virtual dispatch.

### Component(s)

Core