M5 Phase 3: the demo's review arc names a starter, and says nothing when it cannot find it

[kjpatel]

Scoping the last three demo suites turned up the dependency that blocks two of them — and it is exactly the kind this phase exists to remove.

The finding

demo/app.py declares REVIEW_SOURCE_SCENARIO = "notice-ny", plus a matching scenario id, case id and title. The demo is toolkit (D2), so that is a teaching scenario's name compiled into it. _ingest_review_case is wrapped in except Exception: pass, so pointing DULY_DEMO_CONTENT anywhere else makes the arc quietly not appear — no error, no log line, one fewer scenario in a list nobody counts.

Degrading quietly is the right behaviour for a missing demonstration. That is precisely why hardcoding which demonstration went unnoticed: from outside, "this deployment has no review scenario" and "this deployment has one but we only look for notice-ny" are indistinguishable.

Why this PR routes rather than fixes

I started the fix — derive the source from whichever scenario carries a below-floor extraction, preferring notice-ny so this repository's demo stays byte-unchanged — and backed it out. Four constants, nine call sites in app.py, and one constant test_review_arc.py imports.

A half-refactored app.py is worse than a routed finding, and the finding is what the next PR needs to start from. So: a plan task (4b(ii)c-0) carrying the shape of the change, a CLAUDE.md gotcha so it cannot be rediscovered by surprise, and a changelog paragraph on why a silent failure hid a hardcoded name.

Sequencing this settles: test_rules_api (31 failures, the largest) does not touch the arc and can go first. test_evidence_api and test_review_arc both assert on it and cannot.

Also

The one example test in test_content_roots is now labelled as one — its subject is what this repository happens to ship, so it moves with the six packs rather than being repointed. Everything else in that file asserts toolkit behaviour and stays.

Verification

975 passed, 7 skipped
verified 351 cases              # byte-for-byte
git diff -- golden/ fixtures/   # empty

Documentation pass run per CLAUDE.md's definition of done — including the sweep for every doc naming the constants, piped to wc -l rather than head (one hit, the plan).

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