perf(provider): make static-file changeset counts O(files)

[decofe]

Closes RETH-694

changeset_count() scans every .csoff record to compute a progress-bar denominator for the history indexing stages.

• At block 10.7M, that's ~10.7M 16-byte reads per call, taking ~9s
• Both account and storage counts run every pipeline cycle → ~20s total
• This is longer than the batch being processed, so after a restart the pipeline can never catch up

Since .csoff records are cumulative [offset, num_changes], the total is just last.offset + last.num_changes.

• Reads one record per file instead of all records
• O(files) instead of O(blocks) → 22 reads instead of 10.7M

Before: 502ms (2.2M blocks), ~9s (10.7M blocks)
After: 0.2ms (2.2M blocks), ~0.3ms (10.7M blocks)

Repro and timings: https://gist.github.com/yongkangc/06d1da54a32b4a1818c9432483fef7a6

[github-actions]

⚠️ Changelog not found.

A changelog entry is required before merging. We've generated a suggested changelog based on your changes:

Preview

---
reth-static-file-types: minor
reth-provider: patch
---

Added `total_changes()` method to `ChangesetOffsetReader` that efficiently reads only the last sidecar record instead of iterating all records. Refactored `account_changeset_count` and `storage_changeset_count` in `StaticFileProvider` to share a common `changeset_count_for_segment` helper using the new method, and added integration tests for both.

Add changelog to commit this to your branch.

[Copilot]

Pull request overview

This PR optimizes static-file changeset progress denominator computations by avoiding per-record scans of .csoff sidecar files. Instead, it derives the total count per static file from the last committed changeset-offset record, reducing *_changeset_count() from O(blocks) to O(files) while preserving existing stage semantics.

Changes:

• Replace account_changeset_count() / storage_changeset_count() implementations to sum per-file totals computed from the last committed .csoff record.
• Add ChangesetOffsetReader::total_changes() to compute total changes via a single last-record read.
• Add regression tests covering changeset counts across multiple static files for both account and storage changesets.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/storage/provider/src/providers/static_file/manager.rs	Switch changeset count computation to an O(files) approach using a shared helper that reads only the last committed sidecar record per file.
crates/static-file/types/src/changeset_offsets.rs	Add total_changes() API on the sidecar reader plus unit tests validating behavior and committed-length handling.
crates/storage/provider/src/providers/static_file/mod.rs	Add integration-style tests ensuring account/storage changeset counts are correct across multiple static files.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.