Fix the sticking wasm sky: planet-keyed carts in the cruise + a mode-2 rotator

Paul: "the wasm background is sticking — seems to be the same and not shift on the planets." Two holes, one symptom:

• The star-cruise forces the demo layer up via bgWant(), but the only cart rotator (bgFlip) gated on bgMode===1 — in the aliens view the same cart wrapped every planet forever.

• Mode 2 (demos-only) had NO rotator at all — same cart forever on the 2D map too (both the bgFlip gate and the 1s clock's caller gate excluded it).

Verified: test/bg-cart-run.js (mode-2 idle rotation via ?bgAltMs, the cruise stand-down guard, zero page errors). demo-layer-test + full-boot-run green.

▶ Play it: Mercury Interrogation — where the harmony organ has the least anywhere to hide https://stellate.app/?genre=bebop&seed=3324&path=694.13834,713.13867,675.13867&xdur=13.122

8db570b https://github.com/aboard-io/stellate/commit/8db570b45ec3953847b7957036e6549096c7dd93