fix: show contributor rank movement

[rogerchappel]

Summary

• Fix previous-rank lookup so same-day reruns do not compare a leaderboard against itself.
• Add a visible Movement column to contributor leaderboards.
• Repair the latest Australia snapshot so rogerchappel shows the movement from rank 18 to rank 4.
• Repair the latest global snapshot so repeated global contributors show movement against the 2026-05-07 snapshot instead of all appearing as new.

Verification

• Tests or checks run: pnpm run check; pnpm run build; pnpm run validate:data; ad hoc writeSnapshots same-day rerun check returned { "rank": 4, "previous_rank": 18, "delta": 14 }
• Manual review completed: inspected generated Australia leaderboard HTML for the Movement column and rogerchappel showing ↑14; inspected generated global leaderboard HTML for mixed New and movement deltas such as bocaletto-luca showing ↓2 and mP1 showing ↓7.

Risk Level

• Low
• Medium
• High

Notes:

Rendering and snapshot metadata only. No production data mutation, secrets, auth, billing, or public API compatibility changes.

Rollback Plan

• Revert commits 856c1fe and ffea7fc to restore the previous snapshot lookup, table layout, and latest snapshot values.

Human Decision Needed

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

[rogerchappel]

Good