feat(ltl): formula AST and step evaluator for v0.1

Supports Always over Pure/Thunk leaves; eventually/next/bounds
deferred. Once a thunk returns false under an Always, the verdict
latches to violated so the runner can surface the offending step
without later observations masking it.

internal/ltl/formula.go

@@ -0,0 +1,40 @@
+package ltl
+
+import "fmt"
+
+// Formula is the AST of a temporal logic property. v0.1 supports only Always
+// over Pure/Thunk leaves; eventually, next, and bounded operators are
+// deferred to v0.2+.
+type Formula interface {
+	isFormula()
+	describe() string
+}
+
+type AlwaysFormula struct {
+	Inner Formula
+}
+
+type PureFormula struct {
+	Value bool
+}
+
+type ThunkFormula struct {
+	Func func() bool
+}
+
+func Always(inner Formula) Formula { return AlwaysFormula{Inner: inner} }
+
+func Pure(value bool) Formula { return PureFormula{Value: value} }
+
+func Thunk(function func() bool) Formula { return ThunkFormula{Func: function} }
+
+func (AlwaysFormula) isFormula() {}
+func (PureFormula) isFormula()   {}
+func (ThunkFormula) isFormula()  {}
+
+func (a AlwaysFormula) describe() string { return "Always(" + a.Inner.describe() + ")" }
+func (p PureFormula) describe() string   { return fmt.Sprintf("Pure(%t)", p.Value) }
+func (ThunkFormula) describe() string    { return "Thunk(...)" }
+
+// Describe returns a debug-friendly representation of the formula.
+func Describe(formula Formula) string { return formula.describe() }