Wallet reorg, async helper test, drop dead MANIFEST.in / old_tests (#4)

Three independent cleanups bundled into one PR:

1. Move payment_address.{h,cpp} from chain/ to wallet/
   The Knuth C-API has always had `kth/capi/wallet/payment_address.h`
   under wallet/, not chain/. py-native carried the historical
   misclassification. Move both the wrapper header and the wrapper
   implementation to mirror the upstream layout:
     include/kth/py-native/chain/payment_address.h
       → include/kth/py-native/wallet/payment_address.h
     src/chain/payment_address.cpp
       → src/wallet/payment_address.cpp
   Includes in src/module.c and src/wallet/payment_address.cpp,
   plus the source list in setup.py, follow.

2. Salvage the async_exec helper from old_tests/, drop the rest
   old_tests/ had two stale Python "tests" — really standalone
   scripts, not pytest tests:
     - test_native_py: 715 lines, of which the first ~165 dup the
       printers in examples/settings.py, the main() duplicates the
       node bootstrap covered by tests/test_node.py, and the last
       ~550 lines are commented-out JavaScript (a historical
       reference catalog of API calls).
     - test_native_async_py: 210 lines, mostly the same printers
       again, but with a real gem hiding in the middle: an
       async_exec() helper that wraps the kth_native callback API
       (`callback(error, *result)`) into an awaitable via
       asyncio.Future + call_soon_threadsafe. Idiomatic Python
       bridge that downstream consumers (py-api) will want to copy.

   Extract `async_exec` into a new `tests/test_node_async.py` and
   delete old_tests/ entirely. The new file is a *unit* test of
   the helper, not a node test: a single Python process can only
   bootstrap one Knuth node (global logging/thread state from the
   first node_construct/destruct doesn't reset, the second
   construct trips on it), so we drive async_exec with three
   synthetic callable shapes — inline callback, threaded callback,
   error-path — and assert the bridge resolves correctly in each.
   tests/test_node.py keeps covering the live-node path. No new
   dependency: just `asyncio.run()` from a regular sync test, no
   pytest-asyncio needed.

3. Delete MANIFEST.in (redundant + wrong)
   setuptools-scm picks up every git-tracked file for the sdist
   automatically. The MANIFEST.in we had was both redundant and
   incorrect — it referenced version.py (deleted in the previous
   PR), the old chain/payment_address.h location (moved by this
   PR), nonexistent paths (chain/*.h, p2p/*.h, doc/, test/), and
   build artifacts (*.so, *.dll, *.dylib) that have no business
   in the sdist. Drop it. Verified the sdist still ships 111
   files including tests/, examples/, kth_native.pyi, py.typed,
   src/wallet/payment_address.cpp, and the include/wallet/
   directory.

All 6 tests pass locally after the reorg.

Author: fpelliccioni