Rumble scrapper add +1 might to ferrous forerunner and opponent with might 1 play switcheroo to ferous forerunner. How the layering applied?

Ferrous Forerunner ends the turn at 1 Might; the opponent's unit ends at 7.

Layering, step by step:

1. Rumble, Scrapper's "Your Mechs have +1 [M]" is a static passive applied in the Arithmetic layer (CR 479.4.e.1 — increases first). Ferrous Forerunner's current Might is 6 + 1 = 7 (Rule 710).
2. Switcheroo resolves and performs the Swap action (CR 434). It determines the difference between the two units' current Might — 7 vs 1 = 6 — and applies a fixed −6 modifier to the higher unit (Forerunner) and +6 to the lower unit, for the turn (CR 434.1.a–b).
3. The layers loop (CR 478) applies each effect once: 6 (base) + 1 (Rumble) − 6 (Switcheroo) = 1. The opponent's unit goes 1 + 6 = 7.

Rumble's +1 does not re-apply after the swap — it was already baked into the current Might the swap snapshotted, so it doesn't stack a second time on top of the new value. If the opponent's 1-Might unit is itself a Mech, it would start at 2 under Rumble, and the numbers shift accordingly (Forerunner ends at 2, the Mech at 7).

Rule refs: 433.1, 477.3, 710.