feat: add TimeNano field for nanosecond block timestamps

[minhd-vu]

Summary

Add nanosecond-precision block timestamp capture via new TimeNano field in BlockExtraData. This enables tracking precise block propagation timing beyond the second-precision header.Time field.

Changes:

• Add TimeNano field to BlockExtraData (RLP optional)
• Add GetTimeNano() helper on Header
• Add Placeholder hardfork to gate TimeNano validation
• Validate TimeNano presence in verifyHeader for Placeholder+ blocks
• Restructure Prepare to calculate timestamps before BlockExtraData encoding

The Placeholder hardfork name is temporary and will be renamed.

Executed tests

Rollout notes

[codecov]

Codecov Report

❌ Patch coverage is 69.44444% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.45%. Comparing base (89eada7) to head (f1286ea).

Files with missing lines	Patch %	Lines
core/types/block.go	0.00%	12 Missing ⚠️
consensus/bor/bor.go	88.67%	5 Missing and 1 partial ⚠️
params/config.go	42.85%	4 Missing ⚠️

❌ Your patch check has failed because the patch coverage (69.44%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2253      +/-   ##
===========================================
+ Coverage    53.41%   53.45%   +0.04%     
===========================================
  Files          896      896              
  Lines       159745   159770      +25     
===========================================
+ Hits         85322    85413      +91     
+ Misses       69095    69048      -47     
+ Partials      5328     5309      -19     

Files with missing lines	Coverage Δ
params/config.go	37.06% <42.85%> (+0.12%)	⬆️
consensus/bor/bor.go	85.60% <88.67%> (+0.07%)	⬆️
core/types/block.go	41.25% <0.00%> (-1.40%)	⬇️

... and 24 files with indirect coverage changes

Files with missing lines	Coverage Δ
params/config.go	37.06% <42.85%> (+0.12%)	⬆️
consensus/bor/bor.go	85.60% <88.67%> (+0.07%)	⬆️
core/types/block.go	41.25% <0.00%> (-1.40%)	⬇️

... and 24 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[sonarqubecloud]

Quality Gate failed

Failed conditions
12.0% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud

[minhd-vu]

This should be reviewed alongside: #2277