feat(spin-engine): federate multiple physical roots into a single Workspace

[deer]

Engine gains createWorkspace(List<Path>), letting a single invocation span several physical repository roots as one logical Workspace. DefaultEngine validates that the given roots don't overlap (one nested in, or equal to, another) via a new requireDisjoint check, computes their deepest shared ancestor via commonAncestor, and creates a FederatedWorkspace at that synthetic path. Each physical root is then discovered as an ordinary child Project under that shared root, so Project#workspace() and Project#stream() work unmodified across repo boundaries. spin's CLI gains a repeatable --root/-r option (backed by the new build.spin.option.Root) that federates additional roots alongside the working directory.

Along the way this fixes two latent bugs surfaced by the new code paths. DefaultEngine#getWorkspacePath was checking the original path argument instead of the loop's detectionPath while walking up ancestors for Plugin.MetaClass detection, so that branch never actually advanced past the starting directory. Reference#equals compared project().name() while hashCode() used the project's identity hash, violating the equals/hashCode contract; it now compares project() directly, which matters once multiple projects across federated roots can share a simple name.

Coverage includes unit tests for commonAncestor and the disjointness/empty-root edge cases in spin-engine, plus a real integration test in spin-engine-tests (FederatedWorkspaceTests, backed by a new FederatedTestPlugin and two on-disk fixture roots) verifying that two independently-detected roots actually end up as sibling Projects of one Workspace with a unified stream().