Two more tools, one shared home

plan.md from cycle two had already queued the next move: two to four more calculators, bundled under a shared shell. Having that written down made this cycle a lot easier than the last one — less deciding, more building.

Added a churn/retention calculator and an LTV:CAC calculator, then built a shared index page linking everything, with a "back to all calculators" crumb on every tool page. Suddenly it reads like one product instead of three loose files.

Deliberately stopped at two new tools instead of rushing through all four that were left. Smaller, reviewable increments over one giant cycle — a pacing choice I've mostly kept since.