fix(build): build MLX without null-this traps (needed for MLX >= 0.32.1)

[AlexCheema]

Problem

The Nix build of MLX (on macOS) uses nixpkgs' clang 21. metal-cpp, which MLX uses for Metal, calls Objective-C methods through pointers that may be nil, and relies on Objective-C's rule that a message to nil does nothing. For example, NS::SharedPtr's destructor calls m_pObject->release() on a moved-from null. In C++, a member call through a null pointer is undefined behaviour. Where clang 21 can see that the pointer is null, it compiles that path to brk #1. Apple's clang 17, which builds MLX's official wheels, doesn't.

This bites as soon as exo moves past MLX 0.32.0. MLX 0.32.1 (ml-explore/mlx#4211) spreads a model's memory over several Metal residency sets. In ResidencySets::add_set_locked, the fast path of sets_.push_back(Set{std::move(set), 0}) became a trap. The first three sets go through the reallocating path; the fourth finds spare capacity and traps. So loading any model that needs a fourth set, over ~12 GB on a 96 GB Mac, killed the runner with SIGTRAP.

Measured on a Mac Studio (M3 Ultra, 96 GB) with MLX main (0e3ff364), which has the fast-synch deadlock fixes ml-explore/mlx#4552 and #4556:

MLX build	Qwen3.8-27B (4-bit) on one node
this Nix build	runner SIGTRAP on every load attempt, 0/6 requests
official 0.32.3 wheel (same residency code)	6/6 requests
this Nix build + -fno-delete-null-pointer-checks	6/6 requests, 5 residency sets, no crash

Reproduction: a 12-line file, a std::vector of a struct holding an NS::SharedPtr with push_back(Set{std::move(p), 0}). nixpkgs clang 21 at -O3 emits brk #1; Xcode's clang 17 doesn't, and nor does clang 21 with the flag.

Change

Build MLX with -fno-delete-null-pointer-checks (CMAKE_CXX_FLAGS in python/parts.nix), which keeps the semantics metal-cpp depends on.

With the MLX fork exo pins today (based on 0.32.0) this is latent. No runner died with SIGTRAP in any of our chaos runs on it. But any MLX upgrade needs it, and the null-pointer pattern is used throughout metal-cpp.

Tests

• The disassembly of ResidencySets::add_set_locked before and after: the in-place construction is back instead of brk #1.
• On hardware: Qwen3.8-27B loads and serves with MLX's default residency settings, on one node and as a 2-node pipeline.
• CI builds MLX with the flag.

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