Refactor export logic to separate module for better separation of concerns

[coderabbitai]

Problem

Currently, the RPC layer directly uses CLI methods from the archive_cmd module, which violates separation of concerns principles. Specifically, the do_export function in src/tool/subcommands/archive_cmd.rs is being used by both:

• CLI commands (forest-tool archive export)
• RPC methods (Forest.ChainExportDiff)

Proposed Solution

Move the core export logic to a separate crate or dedicated module that can be shared between CLI and RPC layers without creating architectural dependencies.

This would involve:

1. Extracting the core export functionality from archive_cmd.rs
2. Creating a new module/crate for shared export logic
3. Having both CLI and RPC layers depend on this shared module
4. Maintaining clear separation between presentation layer (CLI/RPC) and business logic (export operations)

Context

This issue was identified during review of PR #6074 which adds forest-cli snapshot export-diff functionality.

References:

• PR: feat: forest-cli snapshot export-diff #6074
• Comment: feat: forest-cli snapshot export-diff #6074 (comment)
• Requested by: @hanabi1224

Benefits

• Better architectural separation
• Easier testing of core export logic
• Reduced coupling between CLI and RPC layers
• More maintainable codebase