fix(faigate): restore pydantic-core source-build hardening for clean linkage audit

[typelicious]

Summary

brew upgrade fusionaize/tap/faigate to v2.3.0 prints on every install:

Error: Failed changing dylib ID of /opt/homebrew/Cellar/faigate/2.3.0/libexec/lib/python3.12/site-packages/pydantic_core/_pydantic_core.cpython-312-darwin.so
  from @rpath/pydantic_core._pydantic_core.cpython-312-darwin.so
    to /opt/homebrew/opt/faigate/libexec/lib/python3.12/site-packages/pydantic_core/pydantic_core._pydantic_core.cpython-312-darwin.so
Error: Failed to fix install linkage
Updated load commands do not fit in the header of ... needs to be relinked, possibly with -headerpad or -headerpad_max_install_names

Runtime startup happens to keep working, which masked the regression — but Homebrew's linkage audit is broken on every upgrade and the noisy errors are user-visible.

Root cause

This formula was switched to pip install --prefer-binary (and dropped PIP_NO_BINARY=pydantic-core,watchfiles and the rust build dep) to skip the 3–5 min cargo build. That silently regressed the v1.2.2 hardening recorded in fusionAIze/faigate RELEASES.md. Prebuilt pydantic-core / watchfiles wheels are linked upstream without extra Mach-O headerpad space, so the post-install install_name_tool -id rewrite cannot fit the new install name. The LDFLAGS -headerpad_max_install_names flag the formula kept is a no-op for files pip did not compile.

Fix

Re-introduce PIP_NO_BINARY=pydantic-core,watchfiles, restore the rust => :build dependency, drop --prefer-binary, and inline a comment explaining why so this regression cannot land silently a third time. Same pattern as v1.2.2 / v1.2.3, now mirrored as a golden reference in fusionAIze/faigate Formula/faigate.rb (PR faigate#219).

Trade-off

The 3–5 min source build returns on brew upgrade. This is the price of a clean linkage audit and silent installs for users. Do not switch back to --prefer-binary until pydantic-core upstream ships wheels with sufficient headerpad.

Test plan

• On a fresh macOS arm64 environment: brew upgrade fusionaize/tap/faigate prints zero Failed changing dylib ID / Failed to fix install linkage lines
• faigate --version reports v2.3.0
• brew services restart fusionaize/tap/faigate and confirm the gateway boots and pydantic-validated models work end-to-end
• After merge: bump faigate to v2.3.1 with CHANGELOG note "packaging: restore pydantic-core source-build hardening on Homebrew (regression from v1.2.2 fix)"

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