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

[Sunrisepeak]

Summary

Closes the seven-PR pm subsystem refactor.
Strictly zero behavior change.

• mcpp.pm (src/pm/pm.cppm) is the single import point for callers
  outside the subsystem (cli, build, pack). Re-exports the three
  data-type modules (dep_spec, index_spec, lock_io) but
  deliberately not the flow modules (resolver, commands,
  package_fetcher, publisher) — those remain direct-import-only,
  preserving the encapsulation gained by R1–R6.

• No call-site migration here. Existing import mcpp.lockfile /
  import mcpp.fetcher / import mcpp.publish.xpkg_emit shims keep
  working; new code can reach the same types via import mcpp.pm.

• prepare_dependencies (the high-level operation that should replace
  the ~250-line dep-resolution loop in cli.cppm::prepare_build) is
  intentionally left for a separate PR. Pulling it out is
  behavior-touching and doesn't belong inside the strict-move R-series.

Final pm/ subsystem layout

src/pm/
├── pm.cppm                  (façade, this PR)
├── dep_spec.cppm            (R1)
├── index_spec.cppm          (R1 placeholder, awaits index-config)
├── lock_io.cppm             (R2)
├── package_fetcher.cppm     (R3)
├── resolver.cppm            (R4)
├── commands.cppm            (R5)
└── publisher.cppm           (R6)

Test plan

• mcpp build (worktree)
• mcpp test — 9/9 unit binaries pass
• e2e: 02 / 09 / 12 / 23 / 27 all pass
• Full CI green