*: early fetch on head

[KaloyanTanev]

1. Fetch early not on SSE block, but on SSE head.
2. Clear the saved early fetched attestations on chain reorg.
3. Verify the fetched root from attestation data request is the same as the SSE event.
4. Clear old attestation data.

category: bug
ticket: none

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Pull request overview

This PR changes early attestation-data prefetching to trigger on the beacon node’s SSE head event (instead of the SSE block event), adds verification that fetched attestation data matches the head root from the event, and improves cache invalidation/cleanup to avoid stale early-fetched data.

Changes:

• Switch early fetch trigger from SSE block → SSE head, and pass the head block root through the scheduler → fetcher path.
• Verify early-fetched attestation data votes for the head root from the SSE head event; skip caching on mismatch.
• Invalidate/evict early-fetch caches on chain reorgs and as slots advance; add/extend unit tests for these behaviors.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
core/scheduler/scheduler.go	Updates scheduler wiring to trigger early fetch on head events and propagate the head block root into FetchOnly.
core/interfaces.go	Extends Scheduler/Fetcher interfaces to include the head block root parameter for early-fetch flows.
core/fetcher/fetcher.go	Adds root verification for early-fetched attestation data, stale-slot eviction, and cache invalidation on reorg.
core/fetcher/fetcher_test.go	Extends tests to cover head-root mismatch behavior, reorg invalidation, and stale-slot eviction.
app/sse/listener.go	Emits head events to subscribers with parsed eth2p0.Root block root; stops notifying on block events.
app/sse/listener_internal_test.go	Updates SSE listener tests for the new head-event subscription/notification signature.
app/featureset/featureset.go	Updates feature flag documentation to reflect head-event triggering and root verification semantics.
app/app.go	Wires SSE head events into the scheduler and subscribes reorg events to invalidate the fetcher’s early-fetch cache (when enabled).

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[codecov]

Codecov Report

❌ Patch coverage is 60.41667% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.12%. Comparing base (576c67b) to head (4a62764).

Files with missing lines	Patch %	Lines
core/scheduler/scheduler.go	0.00%	8 Missing ⚠️
app/sse/listener.go	68.42%	3 Missing and 3 partials ⚠️
app/app.go	0.00%	3 Missing ⚠️
core/fetcher/fetcher.go	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4561      +/-   ##
==========================================
+ Coverage   57.08%   57.12%   +0.04%     
==========================================
  Files         245      245              
  Lines       33218    33249      +31     
==========================================
+ Hits        18963    18994      +31     
+ Misses      11867    11859       -8     
- Partials     2388     2396       +8     

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