fix: repair current rank movement data

[rogerchappel]

Summary

• Repair previous_rank metadata across all current country snapshots by comparing each board with its latest prior historical snapshot.
• Remove stale self-comparison values for new entrants so they render as New instead of fake steady/down movement.
• Correct the merged global metadata from PR fix: repair global rank movement data #29 by comparing against the actual 2026-05-12 global snapshot; current global top 20 has no overlap, so those rows correctly render as New.

Verification

• Tests or checks run: pnpm run validate:data; pnpm run build; contributor movement mismatch scan across data/latest/countries-*.json and data/latest/global-contributors.json returned 0.
• Manual review completed: reviewed diff stats and verified commits are split into chunks of no more than 3 files.

Risk Level

• Low
• Medium
• High

Notes:

Data metadata repair only. PR #28 already fixed the weekly script path so future runs skip same-day/current snapshots when calculating prior rank.

Rollback Plan

• Revert this PR to restore the previous current snapshot metadata.

Human Decision Needed

• None
• Maintainer review
• Product/design decision
• Security/privacy review
• Other: