feat: add apple backends and manual platform checks

[zatevakhin]

summary

• add bundled Apple ARM64 CPU and Metal build/link planning, plus explicit external MLX integration with deterministic target, root, file, and deployment validation
• forward Metal/MLX features through both crates and emit target-aware Darwin framework, library, and shared-library link requirements
• add a manual-only hosted platform workflow for exact Rust 1.85.0, Linux ARM64 CPU, macOS ARM64 CPU, Metal compile/link, and Mesa llvmpipe Vulkan backend/op checks

validation

• formatting, clippy, model-free workspace tests, docs.rs-style all-feature checks, warning-denied rustdoc, sys planner/support tests, link modes, packaging, and exact Rust 1.85.0 validation pass locally
• pure planner tests cover Apple CPU/Metal/MLX, Linux ARM64 CPU parity, invalid targets/features, canonical external MLX roots, required files, and Darwin link requirements
• Metal remains compile/link evidence, MLX remains external/manual, Vulkan remains software backend/op evidence, and CPU remains the only supported runtime family