feat(buffa): add MessageFullName trait

[yordis]

• Event-sourced systems identify each event on the wire by its fully-qualified type name (e.g. user.UserCreated). To use a buffa-generated message directly as a domain event. Skipping the usual hand-written mapping layer between "domain type" and "storage type". Callers need that name as a compile-time constant on the type itself. Today it isn't there: the closest surface, ExtensionSet::PROTO_FQN, is bundled with the extension machinery and only emitted when a message opts into unknown_fields=true, which adds a hidden __buffa_unknown_fields field to the struct and forces every struct-literal construction in domain code to trail ..Default::default(). Reaching for buffa::ExtensionSet just to read the FQN therefore drags in storage and ergonomic costs the caller doesn't want.
• Following the same shape as protocolbuffers/protobuf#27111, this PR introduces buffa::MessageFullName as a standalone trait with a single const FULL_NAME: &'static str item. Kept out of Message precisely so the FQN concern stays orthogonal and adding it doesn't break any hand-written impl Message in downstream code. Generic call sites import the trait (use buffa::MessageFullName;) and read T::FULL_NAME at compile time with no runtime indirection, so event stores, type registries, and Any type-URL construction can dispatch on it instead of maintaining a hand-written match arm per message.
• ExtensionSet::PROTO_FQN is intentionally left in place to keep this change additive; the two consts are emitted from the same proto_fqn source in codegen so they cannot drift, and that invariant is documented at both sites.
• The name MessageFullName / FULL_NAME mirrors the convention used across the protobuf ecosystem (and matches protocolbuffers/protobuf#27111 to keep the two PRs aligned). Not attached to it. Happy to rename if a different spelling reads better here.

Related: TrogonStack/TrogonEventStore#240.

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[yordis]

To be extra clear, intentionally opt-out from unknown_fields=true since that causes extra fields that I wish to avoid to codegen, having the trait to a very specific things helps with the Interface segregation principle, which is effectively what is going on for me