feat(printer, config): Add bounded-latency typewriter controller

[JeanMertz]

Introduce a max_latency setting to TypewriterConfig and a matching controller in Printer that automatically reduces the per-character typewriter delay when the queue of pending characters grows large enough to risk falling behind the source by more than the configured budget.

With a fast provider such as Cerebras, the static text_delay / code_delay can cause the typewriter to trail the source by many seconds. Setting style.typewriter.max_latency to, e.g., 200ms keeps the printed output within that window: the effective delay shrinks to min(cap, max_latency / pending_chars) as the queue grows, and expands back toward the cap as it drains — subject to the drain-mode floor described below.

When the provider stops emitting (Event::Finished), the coordinator calls signal_typewriter_drain(), which snapshots the pending- character count. The controller then uses max(snapshot, pending) as the divisor, so the per-character delay never creeps back up as the tail of the queue empties. Any subsequent typewriter enqueue clears the snapshot and returns the controller to live mode.

Setting max_latency = 0 (the default) disables the controller entirely, preserving the existing static-delay behavior.