[Python] Bound Watch state with a timestamp cursor

Opt-in timestamp_cursor=True dedups by a high-water-mark timestamp
instead of by key identity: Watch keeps only the greatest event time it
has emitted for an input and emits the polled outputs strictly past it,
so the per-input state and per-checkpoint encoding are O(1) regardless
of how many outputs the input produces. For sources whose outputs carry
strictly increasing event-time timestamps; the default exact hash dedup
remains for arbitrary-relisting or out-of-order sources.

Also documents the event-time/watermark contract, adds a throttled
late-emission warning, and reuses the parent dedup map on idle rounds so
steady-state empty polls stay O(1) in hash mode too.

Author: Eliaaazzz