[spark] Support time-range incremental batch reads

[Yohahaha]

Summary

Support time-range incremental batch reads in the Spark connector, so pipelines can read only the data written within a [start, end) window (start inclusive, end exclusive).

New options (per-query read options, only read from scan options / TVF args — never from session configuration, so a window can't leak into later reads; batch-only, streaming ignores them):

Option	Default	Meaning
scan.incremental.start.timestamp	(none)	Enables the incremental read; inclusive lower bound. Epoch millis or yyyy-MM-dd HH:mm:ss in the Spark session time zone.
scan.incremental.end.timestamp	latest	Exclusive upper bound; latest stops at the latest committed data captured at planning time.
scan.incremental.timestamp.out-of-range	error	If the start predates the earliest data retained by Fluss (bounded by table.log.ttl): error fails fast (default), adjust clamps to the earliest retained offset.

New TVF fluss_incremental_between_timestamp(table, start[, end]) for pure SQL — sugar over the options above, registered via FlussSparkSessionExtensions; arguments accept string/integral/TIMESTAMP constant expressions (e.g. rolling past hour windows computed in SQL). Note: unlike Paimon's similarly named function, the window is start-inclusive/end-exclusive.

Read semantics per table type:

• Log table: raw records appended within the window.
• Primary key table: keys inserted/updated in the window, folded to their latest value as of the window end (reads only the changelog range, no kv snapshot; keys deleted in the window are excluded).
• Lake-enabled table: same as above, but always reads from Fluss only — an incremental read never unions the lake snapshot.

Also:

• scan.startup.mode is clarified to affect streaming reads only; plain batch reads remain full-table regardless, so default behavior is unchanged.
• Out-of-range end handling: an end timestamp in the future is rejected by the server; empty [start, stop) buckets emit no partitions.
• Docs: website/docs/engine-spark/reads.md (time-range batch read section) and options.md.

Fixes #3842

Test Plan

• New unit tests: FlussOffsetInitializersTest
• New integration tests: SparkTimeRangeTvfTest, SparkLakeTimeRangeReadTest
• mvn spotless:check passes on affected modules

🤖 AI-assisted changes - reviewed by human developer