[BAC-1267] Compile the JSON attribute assigner per exact class

[HolyWalley]

Parsers::JSON generated assign_attributes_from_json via method_missing, defining it on the class of the first instance that parsed JSON. Subclasses inherited that compiled method, method_missing never fired for them, and subclass-only attributes were silently dropped for the lifetime of the process:

Receptionist.from_json({...})                            # compiles assigner on Receptionist
BusyFallback.from_json({'transcript' => 'T'}).transcript # => nil (BusyFallback < Receptionist)

In karen this loses BusyFallback#transcript on every Redis round-trip once a receptionist config has been parsed (any live process), and makes specs order-dependent.

The assigner is now compiled once per exact class — memoized in a class-level ivar, which is not inherited — and dispatched through a stable module method, so a superclass's compiled assigner can never shadow subclass attributes.

Also bumps nokogiri in the dev lockfile (1.16.6 no longer builds on current macOS).