[Doc] CAST has no explicit TimeType branch (Spark 4.1)

## Describe the bug

Spark 4.1 introduced `TimeType` (HH:mm:ss). Spark 4.1's `Cast.scala` adds many `TimeType` arms (`StringType <-> TimeType`, `TimeType <-> TimeType`, `TimeType -> IntegralType / DecimalType`, etc.). `CometCast.isSupported` has no `TimeType` arm at all, so every TimeType-involving cast falls into the default `case _ => unsupported(fromType, toType)` and falls back to Spark.

The fallback is correct, but it is implicit, and `getUnsupportedReasons()` on `CometCast` does not enumerate `TimeType` as one of the unsupported branches, so the auto-generated compatibility guide doesn't mention it.

Surfaced by the cast audit (collection PR queue).

## Expected behavior

Either:

1. Add an explicit `TimeType` arm to `CometCast.isSupported` that returns `Unsupported(Some("Cast to/from TimeType is not yet supported"))`, OR
2. Implement native `TimeType` cast (depends on `TimeType` being supported broadly across Comet, tracked by the broader date/time epic #4418).

The minimum here is the explicit `Unsupported` arm so the compatibility guide reflects it.

## Additional context

- Spark reference: `Cast.scala` in `/tmp/spark-v4.1.1/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/`.
- Comet matrix: `CometCast.scala`.
- Related: #4418 (EPIC for date/time expressions on Spark 4.x).

Start in CometCast.scala and compare its cast handling with Cast.scala from the referenced Spark 4.1.1 source. Trace how unsupported reasons feed the auto-generated compatibility guide, then verify that the guide explicitly reports casts involving TimeType as unsupported without changing native cast behavior.

Written by the indexing model from the issue text.