[P3] rental-shared-picture-set: Studio and Apartment share one decor set where the spec asks for one each

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

GDD mechanic 6 and Epic 5 both say: "Each of Studio and Apartment gets its own wall-palette set and decor/picture set."

Walls are per-kind (STUDIO_WALLS, APARTMENT_WALLS at src/render/pixelSprites/residential.looks.ts:39,43), but both kinds pass the same RENTAL_PICTURES (src/render/pixelSprites/residentialRentalSprites.ts:15,19).

The stated goal behind the requirement ("a strip of the same kind never looks identical") is met, since geoVariant still varies decor within a kind. What is missing is the per-kind split that would make a Studio read as a different class of room from an Apartment at a glance.

Fix when picked up: split RENTAL_PICTURES into a Studio set and an Apartment set. Touches sprites, so it drifts the gallery and needs a pinned-container screenshot refresh.

/gds-code-review.

Start in src/render/pixelSprites/residential.looks.ts and src/render/pixelSprites/residentialRentalSprites.ts, comparing the Studio and Apartment wall and picture-set assignments. Split RENTAL_PICTURES into distinct Studio and Apartment sets while preserving geoVariant variation. Confirm the gallery output and refresh the pinned-container screenshot because the sprite change affects them.