Say what resuming a run's lead is likely to cost, before it is spent

[ntatschner]

team resume now says what the lead's next turn is likely to cost, before anything is spent.

Why

Run 20260923-1216-be60 was resumed at $109.47 of a $120 budget, and its lead spent $15.08 in one turn of 14 exchanges. A long session carries its whole conversation into every exchange. The budget is checked between rounds, and a turn that crosses it finishes: stopping a node mid-turn loses what was already paid for. Checking more often would not have caught a single turn. Nothing said beforehand that this was likely.

What it does

• Where the lead resumes its own session, team resume prints what its last turn cost and over how many exchanges.
• It warns when that is more than the budget has left, or when the budget is already spent.
• A fresh lead gets no figure: it starts small, and the old conversation's cost would overstate it.
• The dashboard's Resume runs the same command, and --dry-run shows the line too.
• The budget check itself is unchanged.

Against the real be60 run, team resume --dry-run now prints:

The lead's last turn cost $15.29 over 2 exchange(s), and it carries that whole conversation into the next, so expect about as much.

Evidence

• ResumeCostTests: five cases, including the be60 figures ($15.08 against $10.53 left).
• Mutation-checked: counting every node's turns, warning against the whole budget rather than what is left, and giving a fresh lead a figure each fail a test.
• Full suite: 3373 passed, 0 failed, 22 skipped.