/launchpad package

NewBPTreeN allocates a BP-tree under /r/gnoswap/launchpad's realm context (the realm that declares Project/Deposit/RewardManager). The tree's PkgID is therefore /r/gnoswap/launchpad, matching the domain values it stores, so tree.Set leaf-slot writes clear the readonly-taint gate regardless of which realm (launchpad/v1, mock, tests) calls Set. Implementations, mocks, and tests must allocate launchpad trees through here rather than calling bptree.NewBPTreeN directly in their own realm.