refactor(pm): add pm/pm.cppm subsystem façade (PR-R7)

src/pm/pm.cppm

@@ -0,0 +1,34 @@
+// mcpp.pm — package-management subsystem façade.
+//
+// This is the single import point that callers in `cli.cppm`,
+// `build/`, `pack/`, etc. should use to reach pm types and high-level
+// operations. The internal modules (`pm.resolver`, `pm.commands`,
+// `pm.package_fetcher`, `pm.publisher`) stay deliberately out of the
+// public surface — re-exporting them would invite call sites to
+// reach into pm internals and undo the encapsulation gained by R1–R6.
+//
+// What this module re-exports:
+//
+//   * Data types only:
+//       - `mcpp::pm::DependencySpec`         (from pm.dep_spec)
+//       - `mcpp::pm::kDefaultNamespace`      (from pm.dep_spec)
+//       - everything currently in pm.index_spec (placeholder; the
+//         index-config implementation will land here)
+//       - `mcpp::pm::Lockfile` and friends   (from pm.lock_io)
+//
+// Internal pm modules (resolver, package_fetcher, commands, publisher)
+// MUST be imported directly by callers that need them; they are
+// intentionally **not** re-exported here. As the call-site migration
+// progresses, an additional high-level operation
+//   `prepare_dependencies(Manifest&, Lockfile&, ...)` → vector<...>
+// will appear here, replacing the ~250-line dependency-resolution
+// loop currently inlined in `cli.cppm::prepare_build`. That landing
+// is tracked separately so this PR stays a pure additive facade.
+//
+// See `.agents/docs/2026-05-08-pm-subsystem-architecture.md` §4.10.
+
+export module mcpp.pm;
+
+export import mcpp.pm.dep_spec;
+export import mcpp.pm.index_spec;
+export import mcpp.pm.lock_io;