fix(prediction): price a gone dispatch at the tariff's own maximum, never below the slot's rate

[chalfontchubby]

🤖 This PR was written by Claude.

Stacked on #5163. It uses the tariff's own import maximum that #5163 works out, so the diff here is only the last commit.

Related to #5392.

Problem

In the PV10 worst case, prediction.py assumes an Intelligent Octopus dispatch slot more than 30 minutes ahead may go away, and prices it at rate_max. rate_max includes any saving session or Axle reward anywhere in the forecast, so an event elsewhere in the day inflates every dispatch slot's worst case. In #5392 a +100p Axle event priced each such minute at 132.25p.

Fix

A gone dispatch now pays the greater of:

• the tariff's own highest import rate, without event rewards (rate_import_tariff_max, kept by set_rate_thresholds());
• the slot's own rate, so an event on that slot still counts, and the worst case can never be cheaper than the nominal case.

rate_scan() resets the tariff maximum to the raw one, so a rescan without fresh thresholds falls back to the old, higher price. A debug replay of a file without the field falls back to rate_max in the same way.

The C++ kernel gets the same change and its parity revision goes to 17. CI rebuilds the checked-in binaries.

This doesn't settle #5392 by itself. That report's root cause is the fixed off-peak minutes being marked as dispatches that may vanish (#5396), which is being reworked separately. This PR only removes the event reward from the worst-case price.

Testing

• test_dispatch_gone_priced_at_tariff_max: the tariff max with an event in the horizon, Prediction uses it, and a rescan resets it.
• test_pv10_dispatch_gone_never_cheaper_than_the_slot: with every slot at 200p and the tariff max at 25p, PV10 costs the same as nominal.
• Kernel parity: an added case with the tariff max below the slot's rate. Python and kernel agree, and removing the floor from the kernel alone fails it.
• ./run_all --quick, --test debug_cases and ./run_pre_commit green.

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