Engine: parameterised programs, values out of band (EN-5) #258

Goal. Parameterised programs: customer values travel out of band from the IQL text, bound by the engine.

Why. Facts and decisions carry customer-supplied values. With parameters, the literal encoder in the gateway and SDKs is no longer the injection boundary.

Done when. execute accepts a program with typed parameters; values never pass through the parser; string, number, NaN and infinity edge cases are refused or encoded exactly (the class of bug fixed in #251 cannot recur); both SDKs and the gateway use it for writes.

Plan reference: EN-5. Security, needed before any hosted use. Part of the Engine production readiness milestone (the current focus; tracker #199). Rule for every engine PR: millisecond query and update latency must not regress, and the perf gate runs before the PR opens.