Add legacy amd64 add-on for older x86_64 CPUs

[davbebawy]

Summary

This adds a manually selectable legacy amd64 variant of Pipecat Assist for older x86_64 Home Assistant hosts that cannot load current NumPy wheels built with the x86-64-v2 baseline.

The standard Pipecat Assist add-on remains unchanged for normal amd64 and aarch64 installs. The new Pipecat Assist Legacy amd64 add-on builds from the same source, but passes LEGACY_AMD64_NUMPY=true and constrains NumPy to <2.0 so older amd64 CPUs have a compatible wheel path.

Background

On older amd64 systems, the standard add-on can crash during startup before Pipecat Assist initializes:

RuntimeError: NumPy was built with baseline optimizations:
(X86_V2) but your machine doesn't support:
(X86_V2).

Home Assistant can distinguish amd64 from aarch64, but it cannot automatically distinguish newer x86-64-v2-capable CPUs from older x86_64 CPUs. Because of that, this PR exposes the compatibility build as a separate add-on entry rather than trying to make Home Assistant auto-select it.

What changed

• Adds addons/pipecat_assist/constraints-legacy-amd64.txt with numpy<2.0.
• Adds LEGACY_AMD64_NUMPY build arg support to the existing Dockerfile.
• Adds addons/pipecat_assist_legacy_amd64 as an amd64-only add-on entry.
• Adds a release workflow job that publishes pipecat-assist-legacy-amd64 from the same source context.

Validation

• YAML parsing passed locally for the updated workflow and add-on metadata.
• This still needs a GitHub Actions image build and a runtime test on affected older amd64 hardware.