feat(builder): compute state root on finalization

[cody-wang-cb]

Summary

• Adds --flashblocks.compute-state-root-on-finalize flag
• When enabled, it will attempt to calculate state root when the build_payload loop exits

Test plan

• Added integration test test_state_root_computed_on_finalize
• Verifies intermediate flashblocks have zero state root
• Verifies final payload has valid (non-zero) state root

2026-01-22T17:48:51.446317Z  WARN Flashblock built flashblock_index=7 current_gas=46170 current_da=0 target_flashblocks=12
2026-01-22T17:48:51.696941Z  WARN Flashblock built flashblock_index=8 current_gas=46170 current_da=0 target_flashblocks=12
2026-01-22T17:48:51.946304Z  WARN Flashblock built flashblock_index=9 current_gas=46170 current_da=0 target_flashblocks=12
2026-01-22T17:48:51.956096Z  WARN Resolve kind Earliest
2026-01-22T17:48:51.975298Z  WARN Finalized payload with state root block_number=50 block_hash=0xdb59c234b3ede63b0f4a8b283d640aec6cdc97d291a81e09f8bb109b7a5255f8 elapsed_ms=18
2026-01-22T17:48:52.013919Z  WARN Fallback block built payload_id="0x03cd2963c629d100"
2026-01-22T17:48:52.015451Z  WARN Flashblock built flashblock_index=1 current_gas=46170 current_da=0 target_flashblocks=12
2026-01-22T17:48:52.194559Z  WARN Flashblock built flashblock_index=2 current_gas=46170 current_da=0 target_flashblocks=12

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[mw2000]

pr looks good to me, think its has a formatting ci failure but should be good to merge after