Replace local TextModel, Backend, Token, Message, and option types with
forge.lthn.ai/core/go-inference. go-mlx is now a pure backend that
registers "metal" into the shared inference registry via init().
Deleted: textmodel.go, options.go, backend.go
Updated: register_metal.go (implements inference.Backend with Available()),
mlx_test.go (uses inference.* types, 4 new tests), go.mod,
internal/metal/generate.go (added RepeatPenalty)
159 tests passing (148 internal/metal + 11 root).
Co-Authored-By: Virgil <virgil@lethean.io>
Tests: MetalAvailable, DefaultBackend, GetBackend, LoadModel error
paths, options, defaults. Model-dependent test skips when model
not available on disk.
Co-Authored-By: Virgil <virgil@lethean.io>
