A test pins a limb's seat: `parseSeat` takes only `top:N` or `<issue>:N`, and moving a limb then restoring its old seat puts the group back #6624

Intent: src/core/limb_seats.ts orders a milestone's limbs and gives the fleet its undo, and no test imports it.

Done:

• [ ] A new test/limb-seats.test.ts. parseSeat accepts top:1 and 42:3, and refuses top:0, 0:1, 01:1, top:, :1, top:1x, top:1234567, a number and undefined.
• [ ] On a test forge with one issue and three limbs: seating reads their order, and moveLimb moves the third to place 1, so the order reads 3, 1, 2. Moving it back to the seat it had restores 1, 2, 3.
• [ ] If a case fails today, the same pull fixes src/core/limb_seats.ts and says so.
• [ ] bun test test/limb-seats.test.ts passes.

Files: test/limb-seats.test.ts, src/core/limb_seats.ts