Document interface upcasts and downcasts

[codeshaunted]

Summary:

• clarify that value.as projects upward to an interface
• clarify that typed match patterns perform downcasts
• add terse examples using the existing Animal types
• bump the plugin patch version to 0.11.1

Validation:

• native as-projection test passed
• native interface match-narrowing test passed
• native interface field-link projection test passed
• integrated example passed