docs/CACHING.md: what /map and home cache, keyed how, invalidated by which writes — with today's numbers #2715

First part of #2675.

Done:

• [ ] a bench (bench/) gives p50/p95 and SQL queries per request for /map and / as the owner and as a stranger, on a fixture the size of this forge; the numbers are in the doc and the pull
• [ ] docs/CACHING.md says for each: what is cached, the key (it includes what the viewer may see, so one person's private data never reaches another), every write that invalidates it, and the max age
• [ ] test: the doc names every write the design lists (a test reads the doc's table and checks those action names exist)