[allformless]

Description

core: use now for DA check instead of chasingHead.time

Rationale

this PR will replace core: reject future chasing heads for DA checks

The previous DA skip condition compared block time against ChasingHead.Time — a
best-known remote head populated from peer sync state via the downloader. This introduced
two problems:

1. Security risk: a malicious peer could advertise a far-future head timestamp to
   suppress DA validation on recent blocks, bypassing blob sidecar checks. A future-time
   guard was added as a mitigation, but it only partially addressed the issue.

2. Unnecessary complexity: ChasingHead was the only consumer of this
   peer-provided head state. The blob retention window (MinTimeDurationForBlobRequests = 18.2 days) is defined in wall-clock time, so the correct reference point is
   time.Now(), not a peer-derived value.

Example

add an example CLI or API response...

Changes

Replace the skip condition:

// before
if block.Time()+MinTimeDurationForBlobRequests < chasingHead.Time { ... }

// after
if block.Time()+MinTimeDurationForBlobRequests < time.Now().Unix() { ... }