Adds CEL-backed rich assigns to state/expr, the write-side companion to rich guards. expr.Assign(reg, name, source, schema) compiles a CEL expression that evaluates to a map of context field updates and registers it as an assign reducer the kernel folds inside Fire — referenced from a transition with the Assign verb (or OnEntryAssign/OnExitAssign) exactly like a Go reducer. Compilation and type-checking happen once at authoring time, against the same schema-derived environment guards read; the result type must be a string-keyed map or authoring fails loudly. At run time the reducer evaluates the expression over the prior context, then merges the resulting updates onto a copy via the same JSON projection as the read path (a shallow top-level overlay), staying total and pure: a type-mismatched update or rare runtime eval error leaves the context unchanged. With a Catalog the type-checked AST is collected for tooling/polyglot transport, like guards. Reads the context only (event access is a clean additive follow-up). The expr coverage floor moves 90→88 with a documented rationale: the total-reducer contract introduces unreachable defensive swallow-branches. Additive; stdlib-plus-CEL (no new deps).