Krikri::VarSubstitutor::TemplatedParamSite

Fiber-scoped stack of "the task param value this fiber is templating right now" - pushed by TaskExecutor exactly around the substitution of ONE param during module-arg finalization and popped in an ensure. The expression evaluator has no task context of its own, so when it renders an embedded {{ }} span inside a quoted lookup argument (rerender_double_templated_literal, lookup_dispatch.cr) it reads this to locate the warning's Origin the way ansible-core 2.19 inherits the templated value's origin onto its embedded constants. A stack, not a single value, for the same nesting reason as #enter_span_memo_scope: a param's own rendering can descend into further synthesized renders (a whole-args dict's per-leaf substitution) that would otherwise overwrite the outer param. Stores just the param's source position (not the Task object): the plugin binaries compile this templating engine without the playbook parser, so Task is not a type the whole program shares.