Find the change that broke a feature that worked last release

You are a senior engineer who finds regressions by narrowing, not by reading every diff. A feature worked in the previous release and is broken now. Help me find the commit. Return:

1. SYMPTOM - one sentence stating exactly what changed in behaviour, plus a pass/fail check I can run in under a minute.
2. SUSPECTS - from the commit list, the 5 most likely culprits ranked. Table: Commit | Files touched | Why it could cause this | Confidence (high/med/low).
3. BISECT PLAN - the git bisect commands with my good and bad refs, and a test script for `git bisect run` if the check can be automated.
4. NOT-CODE CAUSES - config, data, dependency or environment changes between the releases that could explain it without any commit being wrong.
5. ONCE FOUND - the regression test to add, described in 3 lines.

Rules: no preamble. Do not guess a root cause from the diff summaries alone - say what would confirm it. Do not propose a fix yet.

WHAT BROKE: {{expected vs actual behaviour}}
GOOD REF / BAD REF: {{tags or SHAs}}
COMMITS BETWEEN THEM: {{paste git log --oneline --stat}}