refactor(pm): move fetcher.cppm into pm/package_fetcher.cppm (PR-R3)

Step three of the package-management subsystem refactor described in
`.agents/docs/2026-05-08-pm-subsystem-architecture.md`. Strictly zero
behavior change.

* Same shape as PR-R1 / PR-R2: the implementation moves under
  `mcpp::pm` in `src/pm/package_fetcher.cppm`, and the original
  `mcpp.fetcher` module becomes a thin shim that re-exports the new
  module and aliases every public type
  (`EventKind`, `ProgressEvent`, `LogEvent`, `DataEvent`, `ErrorEvent`,
  `ResultEvent`, `Event`, `EventHandler`, `CallError`, `CallResult`,
  `Fetcher`).

* Scope deliberately narrower than the architecture doc's original
  "split into index_repo + index_store + package_fetcher" plan: the
  current `fetcher.cppm` body is a single xlings NDJSON client, not a
  three-way mix. The intended split is for the index-config feature
  (`.agents/docs/2026-05-08-package-index-config.md`) when those
  modules actually need to exist; doing it here would invent empty
  files for shapes that don't exist yet. Filed under "narrow each PR
  to behavior-preserving moves" from the refactor doc §10.

Verification:
* `mcpp build` compiles unchanged.
* `mcpp test` — 9/9 unit binaries pass.
* e2e subset (02 / 09 / 12 / 13 / 23 / 27) all pass.

Author: Sunrisepeak