*: add simplified low latency performance mode

[asddongmen]

What problem does this PR solve?

Issue Number: close pingcap#5705

TiCDC's default timestamp propagation favors throughput and adds avoidable latency for latency-sensitive deployments. SchemaStore can also temporarily cap a dispatcher's resolved-ts; relying only on the next EventStore notification delays recovery.

What is changed and how it works?

• Add opt-in performance-mode = "low-latency"; throughput remains the default.
• Advance LogPuller resolved-ts without the normal batching interval in low-latency mode.
• Shorten downstream dispatcher and Maintainer watermark reporting, and trigger checkpoint calculation when a newer watermark arrives.
• Report the maximum paired per-node resolved-ts lag to avoid an extra sampling-cycle artifact.
• Serialize normal dispatcher scan ownership with a short-lock state machine. Bounded worker queues remain nonblocking and duplicate notifications coalesce; ready, handshake, range calculation, resolved-ts, syncpoint, scan, and send actions run under worker ownership.
• In low-latency mode only, collect schema-blocked dispatchers by keyspace and poll the applied SchemaStore frontier every 50ms. A dispatcher is retried after the frontier advances; a full worker queue retains the blocked entry for the next tick. No SchemaStore notifier or unlimited queue is added.

Check List

Tests

• Unit tests
• Focused race tests
• Manual build and formatting checks

Questions

Will it cause performance regression or break compatibility?

Throughput mode remains the default and does not start schema-blocked polling. Low-latency mode intentionally increases control-message frequency and adds one 50ms keyspace-level check loop; blocked dispatchers consume registry memory until resumed or removed.

Do you need to update user documentation, design documentation or monitoring documentation?

Yes. Document the performance mode and its latency/resource trade-off before GA.

Release note

Add an opt-in low-latency performance mode with faster resolved-ts and checkpoint propagation.