Stale backticked code references outside keel/web (engine._assemble_cts_context, backtest._closed_trade, simulate._slippage_for_products) #982

Found by the PR #978 gatekeeper. These predate #978 and are outside its scope, which is keel.web citations, pinned by test_a_cited_web_module_attribute_exists. They are #967's class of bug in other packages: backticked module.name references that resolve to nothing.

• keel/research/independence.py:60 cites engine._assemble_cts_context. The function is public now, as engine.assemble_cts_context; bias.py and cts_factors.py already cite it that way.
• keel/strategy/stats.py:88 and keel/strategy/rules/base.py:158 cite backtest._closed_trade. keel/strategy/backtest.py defines only _close_trade (line 390).
• keel/commands/slippage.py:169 cites simulate._slippage_for_products. keel/commands/simulate.py has no such function.

agent._DECIMAL_PARAMS (strategy/rules/base.py:349, agent.py:184) is cited deliberately as history ("used to live here") and is not stale.

Fix: correct each reference. Then widen the resolver in tests/web/test_web_prose_stays_true.py from keel.web to these packages, or add a sibling test, so each correction is pinned.

A general resolver over every backticked stem.name is not workable as-is. It hits DB columns, event kinds and config keys, so it needs a module allowlist.