pd: register the class under the external's own name (fixes Pd recursion / can't-instantiate)

[jcelerier]

Symptom

In Pure Data, most objects of an addon like score-addon-puara can't be
instantiated — Pd aborts with its abstraction recursion limit.

Root cause

Pd loads a single-object external by filename and requires the registered
class symbol to equal that filename. avendish names the file and the setup
function after the CMake C_NAME (<C_NAME>.pd_*, <C_NAME>_setup), but the Pd
binding registered the class under symbol_from_name<T>() — the introspected
C++ symbol/identifier (get_static_symbol), not C_NAME.

When they differ — e.g. puara ships C_NAME smoother for a struct Smoother, or
any struct without a c_name meta (so get_c_name/get_static_symbol fall back
to the identifier) — Pd loads smoother.pd_linux, runs smoother_setup, but the
class it registers is Smoother. Pd never finds smoother, retries via the
loader, and eventually hits the recursion limit. (Max has the same class:
class_new(get_c_name<T>()) — see follow-ups.)

Fix

Thread @AVND_C_NAME@ from the setup function into the audio/message metaclass so
class_new() uses the external's own name. Falls back to the introspected name
when the metaclass is constructed directly (tests), so nothing else changes.

Validation

An addon with C_NAME coolthing / struct Foo (no c_name meta) now generates
coolthing_setup() → class_new(gensym("coolthing")) and builds a coolthing.*
external. Before, it registered class Foo and [coolthing] could not be created.

Follow-ups (not in this PR)

1. Max: class_new(get_c_name<T>()) has the same flaw — it should use
   @AVND_C_NAME@ too, or it breaks whenever C_NAME != get_c_name.
2. Help/example patches: generated from the dump JSON's c_name
   (= get_c_name<T>), so they still reference the introspected name rather than
   the shipped C_NAME. Making the dump's c_name authoritative (@AVND_C_NAME@)
   would fix the demo object box in every backend's help patch.

🤖 Generated with Claude Code

https://claude.ai/code/session_014Z4KK2Rays9dTj8J2AJcFZ