ffi 2.5 green: name: Type; field body items in #jni_class

New `JniFieldDecl` AST struct (name + field_type); `JniClassMember`
gains a `field` variant. After consuming a member-name identifier
in the body loop, the parser branches on the next token: `:` →
field path (parse type expr + `;`), `::` → method path (existing).

`static` fields aren't part of the grammar yet and error explicitly
("static fields not yet supported"); only instance fields land here.
Lowering to JNI `Get<Type>Field` / `Set<Type>Field` arrives in 2.13.

124/124 examples green.

tests/expected/ffi-jni-class-05-field.exit

@@ -1 +1 @@
-1
+0

tests/expected/ffi-jni-class-05-field.txt

@@ -1 +1 @@
-/Users/agra/projects/sx/examples/ffi-jni-class-05-field.sx:12:6: error: expected '::'
+parse-only ok