Refactor CometPlainVector by replacing boolean flags with an enum

## Summary

Follow-up to #4364.

`CometPlainVector` currently uses a growing list of boolean fields to dispatch on vector type:

```java
private final boolean isBaseFixedWidthVector;
// (and now implicitly: offsetBufferAddress != -1 as a sentinel for variable-width)
```

As noted by @mbutrovich in the PR review, this pattern doesn't scale well. An enum would make the dispatch explicit and extensible.

## Proposed change

Introduce a small `VectorKind` (or similar) enum, e.g.:

```java
private enum VectorKind {
FIXED_WIDTH,
VARIABLE_WIDTH,
// future kinds...
}
private final VectorKind vectorKind;
```

Replace the boolean flag and the `offsetBufferAddress != -1` sentinel with `vectorKind` checks throughout `CometPlainVector`.

## References

- PR #4364 (comment): https://github.com/apache/datafusion-comet/pull/4364#discussion_r2104...
- Suggested by @mbutrovich, deferred as a follow-up