Let a subclass outside the generated directory win over the generated plugin

Problem/Motivation

The escape hatch for behaviour no structure can express — a subclass in the site's own module — depends on two classes sharing a plugin id and the handwritten one winning. Without that resolution, a site with a subclass gets whichever class the discovery happened to find last.

Proposed resolution

• Implement hook_rest_resource_alter() so that a plugin whose class lives outside the generated directory takes precedence over the generated one with the same id.
• Report the override on the status page — an overridden endpoint that nobody remembers overriding is a debugging trap.
• Fail loudly when two handwritten classes claim the same id: that is a mistake, not a precedence question.
• Functional test with a generated resource and a subclass that changes one key.

Remaining tasks

Everything.