Fix rate text descriptions to use tariff range

[warksit]

Summary

• Rate text descriptions ("cheap", "expensive", etc.) in the status message now compare rates to other rates on the tariff, instead of using the battery charging threshold
• Previously, the charging threshold made most rates appear "expensive" or "very expensive" on multi-tier tariffs like Cosy Octopus, even for the cheapest available slots
• Now the cheapest third of rates are "cheap", middle third "expensive", and top third "very expensive"

Test plan

• Cosy Octopus: 14p = cheap, 28p = expensive, 43p = very expensive
• Agile: rates spread across the three bands proportionally
• Flat rate tariff: shows "fixed"
• Negative/free rates still handled correctly

🤖 Generated with Claude Code

[springfall2008]

This could give a divide by zero error on a fixed rate tariff?

[warksit]

Yes, will add a check for that and resolve the test error (thought I had run them before submitting, sorry)

[warksit]

Thanks for the review. The divide-by-zero was already guarded (the rate_min == rate_max check returns "fixed" before reaching the division), but there was a real bug: I removed the export_cost_threshold local variable definition when switching import to rate_frac, but left the export branch still referencing it — causing a NameError on any export rate lookup. Fixed in f6d4b6c.

Export now uses the same rate_frac approach with 4 bands (very low / low / good / very good, at 0.25/0.5/0.75 boundaries). Also added dedicated band_rate_text tests covering flat rate, Cosy Octopus, and Flux tariffs (import + export).

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.