Watch prompt caching hit and miss

The experiments only show what you expect if the lease sits where the cache can find it. Prompt caching matches whole content blocks, in order, and a -p prompt arrives as one block, so "lease followed by question" in -p changes as a whole every time the question does. The decision that matters most is therefore the layout: the fixed instructions and the lease go in the system prompt (passed as a file), and only the question goes in -p. Experiment e then tests the exercise's original layout instead of taking that claim on trust.