eventservice: backport adaptive scan window to release-8.5

[ti-chi-bot]

What problem does this PR solve?

Issue Number: ref #4172, ref #4365, ref #5041, ref #5394, ref #5546

The adaptive event-service scan-window feature is a complete sequence of changes. Backporting only its first PR would omit stability fixes, barrier progress fixes, and the rollback switch required for safe use on release-8.5.

What is changed and how it works?

This PR backports the complete series #4030 → #4460 → #4950 → #5395 → #5547 to release-8.5:

• *:improve memory control #4030 introduces memory-aware scan windows, congestion feedback, metrics, and the DDL workload.
• eventservice: fix changefeed getting stuck  #4460 lets dispatchers with pending DDL barriers advance when the global window is pinned.
• eventservice: optimize scanwindow #4950 replaces the initial policy with the stable adaptive pressure controller.
• pkg/eventservice: add scan window switch #5395 adds debug.event-service.enable-scan-window, enabled by default.
• eventservice: advance scan window for pending syncpoints #5547 extends local progress to pending syncpoint barriers.

The backport also resolves release-8.5 conflicts while preserving its newer quota-release, filter-lifecycle, heartbeat-epoch, and dispatcher-metric fixes. The dashboard layout is adjusted to avoid overlapping panels.

Check List

Tests

• Unit test
• Manual test (source PR *:improve memory control #4030: 2-hour pause, syncpoint enabled, sustained DDL, and delayed MySQL sink)

Local validation:

• make unit_test_pkg for eventservice, config, common event, eventcollector, dispatcher, and dynstream
• cd tools/workload && go test ./...
• make fmt
• make cdc
• make check

Questions

Will it cause performance regression or break compatibility?

No compatibility break is expected. The scan-window switch defaults to enabled and can disable the feature for rollback.

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

No user or design documentation update is required. Event-service monitoring dashboards are updated.

Release note

Backport adaptive event-service scan-window control and barrier progress fixes to TiCDC 8.5.

[ti-chi-bot]

@asddongmen This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[ti-chi-bot]

Cherry-pick conflicts appear resolved; removing the do-not-merge/hold label.

[asddongmen]

/test all

[asddongmen]

/retest

[asddongmen]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: asddongmen, lidezhu

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Needs approval from an approver in each of these files:

• OWNERS [asddongmen,lidezhu]
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