update minimum go to 1.26

[acoshift]

Raise the module floor to Go 1.26 and modernize the library to current Go idioms.

Toolchain

• go.mod and CI matrix: 1.23 → 1.26
• CI actions: actions/checkout@v5, actions/setup-go@v6, codecov/codecov-action@v5

Modern Go

• errors.AsType for OKError / *ProtocolError classification
• reflect.TypeFor instead of comparing Type.String() for handler injection
• skip grpc call options with Type.IsVariadic() instead of “last arg is a slice”
• strings.SplitSeq for SSE data lines, for i := range n, any, reflect.Pointer
• README example uses am.Handler (the old am.Handle did not exist) and a method-aware ServeMux pattern

Behavior changes (v2)

These are intentional, tested, and should be treated as runtime contract updates, not style-only:

1. Wrapped errors. EncodeError walks the unwrap tree. fmt.Errorf("%w", NewError(...)) is now 200 with the inner JSON; wrapped ErrNotFound is now 400. Previously both were 500 {}.
2. OKError embeds error. Required for errors.AsType[OKError]. Existing implementers already have Error() because they are returned as error.
3. WrapError does not re-wrap a value that already unwraps to OKError or *ProtocolError (including custom OKError from UnmarshalForm).
4. Non-variadic last []T is a real request parameter. Only a variadic last arg is skipped as grpc options. func(items []int) now decodes JSON instead of panicking on Call.

errors.Join still prefers the first OKError in the tree (AsType order).