[P3] rental-star-coupling-untested: Epic 7's star half has no test

Deferred from the /gds-code-review Acceptance Auditor on PR #633 (Modern rental living, v2.0.0).

Epic 7's done-when reads: "a tower that loses an Apartment floor shows a population/star dip that recovers on re-lease, a Studio-only tower's stars stay stable through scrappy conditions, and a test pins both."

Only the population half is pinned. src/engine/rentalLiving.test.ts:345-355 asserts a population delta; star appears in that file only as fixture setup (lines 35, 63-68). There is no star-dip test, no recovery-on-re-lease test, and no Studio-stability test anywhere in the branch.

The behavior is believed correct (rentals feed the same population census the star rating reads), so this is a coverage gap rather than a known defect. It is filed because the epic explicitly asked for the test and the epic was marked complete.

Fix when picked up: add the two assertions Epic 7 names.

/gds-code-review.

Start in src/engine/rentalLiving.test.ts, especially the existing population assertion at lines 345-355 and the star fixtures at lines 35 and 63-68. Run the rental living tests, then add coverage for the Apartment star dip and recovery on re-lease plus Studio-only star stability. Done means the Epic 7 star behavior is pinned by tests.