feat(static-file): incremental changeset offset storage

[gakonst]

Summary

Replace inline Vec<ChangesetOffset> in SegmentHeader with a separate .csoff sidecar file for incremental append/prune operations.

Problem

Previously, changeset offsets were stored as Vec<ChangesetOffset> in SegmentHeader and fully rewritten on every commit. For segments with 500k+ blocks, this meant ~8MB rewritten per commit even when appending a single block.

Solution

• Store offsets in a separate .csoff sidecar file (fixed 16-byte records)
• SegmentHeader now stores only changeset_offsets_len: u64 (count)
• Crash consistency: sidecar is synced before header commit

Performance Impact

Operation	Before	After
Append 1 block	O(total_blocks)	O(1) (16 bytes)
Commit overhead	~8MB (500k blocks)	~100 bytes
Prune	O(remaining_blocks)	O(1)
Lookup	O(1) from memory	O(1) from mmap/pread

Changes

reth-static-file-types

• Add ChangesetOffsetsWriter/ChangesetOffsetsReader for sidecar I/O
• Replace changeset_offsets: Vec with changeset_offsets_len: u64 in SegmentHeader
• Update serialization to write u64 count instead of Vec

reth-nippy-jar

• Add changeset_offsets_path() for .csoff file paths
• Update delete() to clean up sidecar files

reth-provider

• Add ChangesetOffsetsWriter to StaticFileProviderRW
• Track current block offset during writes, write to sidecar on block completion
• Sync sidecar before header commit (crash consistency)
• Add read_changeset_offset()/read_changeset_offsets() to StaticFileJarProvider
• Update manager.rs to read offsets from sidecar file
• Update truncate_changesets to truncate sidecar file

Breaking Change

This changes the static file format for changeset segments. Existing changeset static files are not backwards compatible and must be regenerated.

Testing

cargo test -p reth-static-file-types
cargo check -p reth-provider

cc @joshieDo @mattsse

[gakonst]

CI Fix Blocked

The base commit (feat: sparse trie as cache #21583) introduced inconsistencies in SegmentHeader:

1. The struct uses changeset_offsets: Option<Vec<ChangesetOffset>>
2. But methods like changeset_offsets_len() try to access self.changeset_offsets_len (a non-existent field)
3. Tests use changeset_offsets_len: 100 but struct expects changeset_offsets: Some(vec![...])

Options:

1. Fix the base commit first (separate PR to fix feat: sparse trie as cache #21583's breakage)
2. This PR should include fixes for the base commit inconsistencies

Closing this draft PR until the base is stable. Will re-open with a clean implementation.

[gakonst]

Closing due to inconsistencies in base commit. Will re-open after fixing upstream issues.