Export a formal HistoryProvider Protocol from op_system's public API

[jc-macdonald]

Motivation

op_system's convolve_history/history_eval_fn machinery has a real, enforced runtime contract: callers pass a history_provider object whose .query(signal_id, body, **options) method gets forwarded as __hist_query inside the compiled history-aware closure (compile.py:1279-1318). This is documented today only as README prose ("Runtime contract from lowering: __hist_query(signal_id, body, **options)") and typed as bare history_provider: object on every public signature that accepts one (CompiledRhs.history_eval_fn, PytreeEvalFn-adjacent wrappers, etc.) — there is no actual Protocol class in the public API to type-check an implementation against.

This matters concretely now: ACCIDDA/op_engine#67 (revised) needs to port ACCIDDA/COVID19_USA's _history.py — a ~400-line fixed-cadence ring-buffer DDE/history provider — into op_engine as the backing implementation for diffrax-based adaptive methods. Without a formal HistoryProvider Protocol exported from op_system, that port has nothing concrete to implement against beyond README prose, and there's no structural protection if the implicit contract drifts as op_system's lowering evolves (e.g. if query's keyword-only options shape changes, nothing would catch a mismatch until a downstream runtime failure).

Scope

• Define class HistoryProvider(Protocol): def query(self, signal_id: int, body: Array, **options: object) -> Array: ... (exact signature to be confirmed against the real call site at compile.py:1289) in _typing.py (alongside the existing Array protocol) or a new small module.
• Export HistoryProvider from op_system.__init__'s public __all__.
• Update CompiledRhs.history_eval_fn and any other history_provider: object-typed signatures to use the new Protocol instead of bare object.
• Update the README's runnable convolve_history example (ZeroHistoryProvider class) to reference/type-annotate against the new Protocol.

Acceptance criteria

• HistoryProvider importable from op_system.
• history_provider parameters across the public API use the Protocol type instead of object (narrows an instance of the broad-object-typing pattern already tracked generally by Tighten backend typing in compile API (replace broad object usage) #78, specifically for this one parameter).
• mypy --strict passes for touched modules.
• Existing history/convolve_history tests unaffected (this is a typing-only change, no runtime behavior change).

Relationship to other issues / repos

Directly motivated by ACCIDDA/op_engine#67 (diffrax/lineax-backed adaptive methods, needs to port a HistoryProvider implementation) and ACCIDDA/op_engine#26 (tracking). Narrower, protocol-specific instance of the broader typing cleanup tracked by #78.