refactor: allow fetching easily first da included height

[julienrbrt]

Overview

Closes: #2117

• Remove unused genesis helpers (execution environment should do that)
• Rename GenesisDaStartTime to simply StartTime, as it is effectively the network start time
• Add rpc to fetch first da included height
• Save first da included height in a separate key
• Rename some confusing terms and add go doc.

Execution environments will then be able to poll that data from the nodes if wanted (no need to do that here, can be done in at genesis parsing).

[github-actions]

The latest Buf updates on your PR. Results from workflow CI and Release / buf-check (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	⏩ skipped	✅ passed	✅ passed	Sep 1, 2025, 9:09 AM

[codecov]

Codecov Report

❌ Patch coverage is 77.08333% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.07%. Comparing base (a2714aa) to head (d41a832).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
block/manager.go	54.54%	8 Missing and 2 partials ⚠️
block/da_includer.go	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2614      +/-   ##
==========================================
- Coverage   71.13%   71.07%   -0.06%     
==========================================
  Files          73       73              
  Lines        7631     7644      +13     
==========================================
+ Hits         5428     5433       +5     
- Misses       1750     1756       +6     
- Partials      453      455       +2     

Flag	Coverage Δ
combined	71.07% <77.08%> (-0.06%)	⬇️

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[tac0turtle]

much better UX than looking at DA for the info