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

[andygrove]

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 [EPIC] Implement all Spark date/time expressions #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: [EPIC] Implement all Spark date/time expressions #4418 (EPIC for date/time expressions on Spark 4.x).