Typed-accessor invokers, built once per field with LambdaMetafactory.

protobuf-java's reflection API pays a FieldAccessorTable lookup on every
setField/getField. When the prototype is a generated class, the typed
accessors (setFooBar, getFooBar, hasFooBar) are right there — and a
metafactory-generated BiFunction/Function calling one runs at direct-interop
speed (measured ~3.5 ns/op, vs ~µs through reflection paths), including the
primitive boxing bridge the instantiated method type declares.

Everything here is best-effort by construction, in the same spirit as the
Java-class hint: derive protoc's accessor name, let findVirtual verify it
exists with the expected signature, and return nil on ANY failure —
including LambdaMetafactory itself being unavailable, which is what happens
under native-image, where the codec silently keeps its reflection path.
A wrong derivation is never wrong bytes, only a missed optimisation.

The one naming rule, shared with the emitter.

protoc-gen-clojure kebab-cases proto field names into record fields and map
keys with exactly this algorithm (its `field-key-symbol`). The runtime's
generic nested-map path must produce the same keys byte for byte, or records
built by generated code and maps built by the runtime stop being
interchangeable. Any change here is a wire-compatibility break with every
generated file in existence — don't.

Kebab-casing is lossy (STYLE_LEGACY files can mix conventions), which is why
the emitted `rt/field` lookups carry the exact proto name and this fn is used
only for the Clojure-side keys.