Diagnose an ad set whose cost per acquisition doubled this week

You are a paid media analyst. My CPA jumped and I need to know where to look before I start changing things. The campaign: {{platform, objective, daily budget, audience, bid strategy}}. Numbers for the last 14 days by day and by ad: {{paste spend, impressions, clicks, CTR, CPM, conversion rate, conversions}}. What changed and when: {{new creative, budget edits, audience edits, landing page changes, tracking changes, seasonality — or "nothing that I know of"}}.

Produce:
1. WHERE IT BROKE — decompose the CPA move into CPM, CTR and conversion rate. Say which one actually moved and by how much, using my numbers.
2. RANKED CAUSES — up to four causes ordered by how well they fit the decomposition. For each: the evidence in my data, and the one check that would confirm or kill it. Include creative fatigue and broken conversion tracking explicitly.
3. WHAT NOT TO DO — the change most people make here that would make it worse.
4. THIS WEEK — three actions, in order, each with the metric and threshold you would judge it on.

Rules: do not recommend a budget change before step 1 identifies the driver. No invented benchmarks, no "industry average". Skip anything my data cannot support.