Publish a plain-language changelog for param.rs changes

Problem docs/BIDIRECTIONAL_BOOST_CHANGE.md says future algorithm updates will be understandable "by checking the diffs". In practice the diffs are hard to follow:

• 17 commits touched home-mixer/params/param.rs since 2026-08-01, all titled "Open-source X Recommendation Algorithm"
• The repo has 0 releases and 0 tags
• Every sync rewrites the // ... last sync <timestamp> header, so some commits change nothing but that line, while others change ranking weights with the same commit message

Example: b089ce6 (Aug 17) → 6bb4594 (Sep 12), param.rs only

• VqvWeight 0.05 → 0.0
• DwellWeight 0.0 → 0.05
• Two mode strings DENSE_WITH_SHORT_DWELL → DENSE_WITH_LONG_DWELL
• New params such as WeightPerturbationSigma and EnablePhoenixScoreStatsExperimentBucket

None of these are described in a commit message or doc.

Proposal

1. Generate a param changelog on each sync (commit body or docs/PARAM_CHANGES.md): name, old value, new value, date.
2. Skip commits whose only change is the sync header, or label them as such.
3. Tag each sync so a change can be referenced by version.
4. For ranking-weight changes, add a one-line plain-language note in the style of BIDIRECTIONAL_BOOST_CHANGE.md.

Why People currently reverse-engineer diffs by hand, and third-party readings fill the gap, often wrongly. A changelog makes the transparency this repo is built for reach the creators it affects.