Split client.py into session, account, orders and push layers (no behaviour change)

[Bogzx]

Stacked on #3. Merge #3 first. If GitHub doesn't retarget this PR automatically after #3 merges, run gh pr edit 4 --base main.

Splits the 1,430-line client.py into layered modules. No behaviour change.

Why

client.py was a single class holding the HTTP plumbing and error mapping, login, keepalive and re-login, account reads, order placement (including IF-THEN SL/TP groups and conditional modify) and the Push API with its channel class. Reviewing or changing any one of these meant reading all of it.

What changed

The client is now a stack of layers that all operate on the same object, one module each:

Module	Layer	Contents
_session.py	_SessionLayer	_url, _request, error mapping, login/logout/ping, keepalive, re-login, parse_interval
_account.py	_AccountLayer	get_accounts, get_balance, get_positions, get_orders, get_order_history, …
_orders.py	_OrdersLayer	place_order, modify_order, cancel_order, close_position, close_all, modify_position_sl_tp
_push.py	_PushLayer, _PushChannel	websocket channels, subscriptions, message routing
_state.py	_ClientState	the shared attributes, declared once so each layer type-checks on its own
client.py	DXTradeClient	constructor, repr, context manager; re-exports parse_interval and _PushChannel

• Method bodies moved unchanged, with one layering fix. The session layer's logout() used to call disconnect_websocket() from the push layer. Now _PushLayer.logout() closes the channels and then calls super().logout(), in the same order as before.
• One test changed: tests/test_session.py patches time.sleep through dxtrade_wrapper._session instead of dxtrade_wrapper.client. That patches the same global time module.

Verification

• python -m pytest -q: 127 passed, 13 skipped, the same as Prepare 0.2.0 for PyPI: self-contained sdist, settled public API, Trusted Publishing release workflow #3. DXTRADE_SPEC_TESTS=1 pytest tests/test_spec_conformance.py: 13 passed. ruff: clean. mypy dxtrade_wrapper: no issues in 9 files.
• API-surface diff against the original class, loaded from a saved copy of the old client.py:
  • no member added or removed;
  • identical signatures for every method;
  • the two properties (is_authenticated, _ws_connected) have byte-identical source;
  • the only docstring change is logout, now split across two layers.
• examples/offline_demo.py runs end to end and issues the same 11 requests as before.
• python -m build then twine check --strict: PASSED. The wheel contains all new modules plus py.typed.

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