Backend fixes: Pd message-object hot inlet, Python stable ABI, Max CRT guard

[jcelerier]

Fixes to the Pd, Python and Max backends. Validated with a full local build of a 26-object add-on across all three backends (Windows / MSVC, CPython 3.13).

Pd — restore the message object's hot inlet

A message object's leftmost inlet is hot: a message sent to it should set the first parameter and run a processing pass so the object emits output (the Max backend now does the same). process_inputs() was consuming every parameter message — the first inlet's included — as set-only and returning before default_process() could run the object, so the hot inlet produced nothing. process_inputs() gains a skip_first flag so a message to the first parameter falls through to default_process() (set + process + output); cold inlets stay set-only.

Validated: a message to inlet 0 with no trailing bang now produces output; the pre-fix build produces nothing.

Max — register objects under the external filename, and run the hot inlet

Two issues that left most Max objects unusable:

• Loading: ext_main registered the class under the object's c_name() metadata, but Max looks an object box up by the external's filename. When they differ the file loads yet no class of that name exists, so the object never instantiates. The external's C_NAME is now passed into the metaclasses (as Pd already does) and used for class_new; the audio and jitter metaclasses take the same parameter so ext_main passes it uniformly.
• Hot inlet: a named message on inlet 0 was consumed as set-only, returning before the object ran, so a [Name v1 v2( message produced no output. It now runs a processing pass after setting, like the other inlet-0 paths.

Python — opt-in stable-ABI build; use Development.Module

Adds an opt-in stable-ABI build (AVND_PYTHON_STABLE_ABI, OFF by default) that links the limited API (Py_LIMITED_API, python3.dll) via Python_add_library(USE_SABI). It is off by default because pybind11 uses the full CPython C API and does not compile under Py_LIMITED_API, so the stable-ABI path is only usable with a limited-API-capable binding; when enabled it needs the Development.SABIModule component to link python3.lib, and the decision is cached so avnd_make_python() (called from the consumer's scope) sees it. The default keeps the working version-specific build; switching from full Development to Development.Module also stops the module linking the interpreter library.

Validated: stable ABI on → link switches to python3.lib but pybind11 fails to compile (as noted); default version-specific build imports cleanly on the CPython it is built against (26/26 on 3.13).

Max — warn when the static-CRT request is silently dropped

MSVC_RUNTIME_LIBRARY (/MT, required by the Max SDK) only takes effect when CMP0091 is NEW in the scope creating the target, which a function here cannot change. Emit a warning at target creation when it is not NEW, so a consuming project that pins an older policy — and would silently ship /MD externals that crash Max — is caught. (The corresponding consumer-side fix is to require CMake >= 3.15.)

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