Pipeline behavior ordering previously depended on discovery/registration
order. Add an AddBehavior<T>(order:) API that captures the order at
registration time; the BehaviorTypeIndex now orders matching behaviors by
that explicit order (lower runs first), falling back to registration order
for ties. Removed the runtime Order property approach.
Handler/behavior resolution previously forced closed generics before open
generics regardless of registration order, which was surprising and fragile.
TypeIndex now tracks each registered type's index and orders matching
candidates by registration order, so closed/open ordering is deterministic
and follows registration.
Multiple handlers for the same request were previously resolved via .First(),
with ordering depending on Type.GetInterfaces() order (non-deterministic).
Now ScalarRequestInvoker/StreamRequestInvoker throw InvalidOperationException
listing the candidate handlers when more than one matches.
Validation.IsValid previously counted every problem regardless of severity,
leaving Severity inert. Now IsValid only fails on Error problems, so Warning
and Info no longer invalidate the result, and IsValidFor(Severity) lets
callers set the failing-severity threshold.
Rule<T,TProperty>.Validate cast a null instance to T and validated it, so a
reference T whose accessor dereferences the instance threw
NullReferenceException on a direct validator.Validate(context) with a null
instance. Guard the null-instance path and return no problems instead.
Passing a null bound to GreaterThan/LessThan/Between etc. called
value.CompareTo(null), throwing NullReferenceException for non-null
reference-type values. Short-circuit when the bound is null so the rule is
satisfied instead of crashing.
Result/Result<T> equality treats any two failures as equal regardless of their
Error, which was undocumented and surprising for failure-specific branching.
The doc comment referenced `Geekeey.Extensions.Result.Prelude` but the actual
namespace is `Geekeey.Request.Result`, so the suggested using directive would
not compile.
