fix: use smaller skip length for tipset_by_height cache

[hanabi1224]

Summary of changes

Changes introduced in this pull request:

• use smaller skip length for tipset_by_height cache to match Lotus for better RPC performance

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

Outside contributions

• I have read and agree to the CONTRIBUTING document.
• I have read and agree to the AI Policy document. I understand that failure to comply with the guidelines will lead to rejection of the pull request.

Summary by CodeRabbit

• Chores
  • Increased internal lookup cache capacity to improve responsiveness and reduce lookup latency.
  • Standardized epoch checkpointing to a fixed interval for more consistent and efficient epoch processing.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 3dd70da7-1f96-45ac-965a-e881863513c6

📥 Commits

Reviewing files that changed from the base of the PR and between adfa0a5 and 74bbc5c.

📒 Files selected for processing (1)

• src/chain/store/index.rs

🚧 Files skipped from review as they are similar to previous changes (1)

• src/chain/store/index.rs

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Walkthrough

The PR increases the tipset-height LRU cache size and replaces use of CHAIN_FINALITY for checkpointing with a fixed CHECKPOINT_INTERVAL: ChainEpoch = 20, updating related alignment/detection logic and comments.

Changes

Cohort / File(s)	Summary
Cache & checkpoint config src/chain/store/index.rs	Increased ts_height_cache capacity from 20480_usize to 1048576_usize. Replaced dynamic CHAIN_FINALITY-based checkpoint interval with a fixed CHECKPOINT_INTERVAL: ChainEpoch = 20, adjusted next_checkpoint and is_checkpoint logic, and updated comments.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Suggested labels

RPC

Suggested reviewers

• LesnyRumcajs
• akaladarshi

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: using a smaller skip length (fixed CHECKPOINT_INTERVAL of 20 instead of CHAIN_FINALITY) for the tipset_by_height cache lookup optimization.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch hm/use-small-skip-length

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch hm/use-small-skip-length

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.56%. Comparing base (160617b) to head (74bbc5c).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/chain/store/index.rs	85.79% <100.00%> (ø)

... and 13 files with indirect coverage changes

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