docs: add conflict handling and FRI guidance

[westonpace]

Summary

• Add a "Conflict Handling" section to the performance guide explaining how concurrent operation conflicts affect throughput, with common conflict examples and a link to the transaction spec
• Add a "Fragment Reuse Index" subsection to the performance guide describing how the FRI avoids compaction/index conflicts, with a Python API snippet
• Add an "Impacts" section to the FRI spec covering conflict resolution changes, index load cost, and FRI growth/cleanup

Test plan

• Verify doc links resolve correctly (conflict resolution anchor, FRI spec link)
• Review rendered markdown for formatting

🤖 Generated with Claude Code

[github-actions]

Review

Docs-only PR — content is clear, well-structured, and accurate.

One minor nit: Both frag_reuse.md and the original file end without a trailing newline (no newline at EOF). This is a common source of noisy diffs later. Consider adding a final newline to frag_reuse.md.

Otherwise LGTM.

[Xuanwo]

Thank you!