Add partial type stubs for core modules

[heejaechang]

Summary

• add complete sidecar stubs for hupper.interfaces, hupper.logger, hupper.polling, and hupper.utils
• ship the PEP 561 py.typed marker in wheels and sdists
• preserve each touched module's complete public runtime surface, including real reexports, mutable fields, nominal ABC/Thread inheritance, parameter names, and defaults

This is intentionally partial. hupper.watchdog remains unstubbed because its real platform-selected watchdog.observers.Observer base is not a legal static base alias and its concrete external bases remain partially unknown under unfiltered VerifyTypes. ipc/worker/reloader also remain unstubbed because the Windows IPC surface publicly reexports hupper.winapi, including the real partially-unknown ctypes.WinDLL object. Hiding or replacing either boundary would misrepresent the runtime API.

Type completeness

Exact unfiltered pyright --verifytypes hupper --outputjson, without --ignoreexternal:

Environment	Before	After
watchdog 6.0 installed	29.20% (80 known / 23 ambiguous / 171 unknown; 274 total)	48.43% (139 known / 20 ambiguous / 128 unknown; 287 total)
watchdog absent	29.20%	48.43%

The 13 added denominator symbols are real runtime reexports made explicit by the complete touched-module stubs; no exports were invented or hidden. Every touched export is known.

Validation

• strict Pyright and mypy compile all four stubs and positive consumers
• adversarial consumers reject wrong callback arity, invalid factory returns, incomplete monitors, nominal logger impostors, and streams without isatty
• exact runtime/stub comparison passes for module names, class members, instance/class fields, signature names/kinds/defaults, and MROs
• unflagged scoped python -m mypy.stubtest passes all four modules in fresh wheels with watchdog installed and absent
• full stubtest reaches only documented untouched source errors (watchdog.Observer, conditional winapi/ipc, and removed distutils)
• tox format, lint, check-manifest, build, twine, and coverage tasks pass
• Python 3.9, 3.12, 3.13, 3.14, and 3.15 produce the same clean-main result: 8 tests pass and 2 existing integration tests fail because current pytest-cov no longer provides pytest_cov.embed
• fresh installed wheels pass polling delivery, logger/ABC checks, and a real spawned subprocess IPC round-trip, both with watchdog installed and absent
• wheel and sdist contain exactly the four .pyi files plus py.typed; runtime .py hashes are unchanged

Local limitations: Python 3.7, 3.8, 3.10, 3.11, PyPy, and POSIX were unavailable. Pyright's Python 3.7 mode accepts the stubs; mypy 1.4.1 accepts the four stubs for Python 3.7. The docs tox environment matches clean main by failing only because POSIX make is unavailable on this Windows host.