Implement native macOS environment access for amd64 and arm64

## Summary

The macOS amd64 and arm64 `std::sys::env` providers currently validate their arguments and then return `ERR_NOT_IMPLEMENTED`. As a result, the portable `std::env` API compiles on macOS but cannot read the process environment.

## Relevant code

- `std/sys/macos/amd64/env.wave`
- `std/sys/macos/arm64/env.wave`
- `std/env/environ.wave`

## Scope

Implement a native Darwin environment provider for both supported macOS architectures. Use a documented process environment entry point or a narrowly scoped system-library adapter; do not make high-level `std::env` depend directly on `std::libc`.

## Acceptance criteria

- `env_read` returns the process environment as the NUL-separated format expected by `std::env`.
- amd64 and arm64 use the same public provider contract.
- Invalid pointers and capacities retain deterministic negative errors.
- Buffer exhaustion is distinguishable from an empty or unavailable environment.
- Native macOS CI verifies a known injected variable and a missing variable.
- No `ERR_NOT_IMPLEMENTED` path remains for normal environment access.

Start with std/sys/macos/amd64/env.wave and std/sys/macos/arm64/env.wave, then compare their provider contract with std/env/environ.wave. Identify the documented Darwin environment entry point or narrowly scoped system-library adapter, preserving the stated error distinctions. Done means both architectures provide NUL-separated environment data, native CI checks present and missing variables, and no normal-access ERR_NOT_IMPLEMENTED path remains.

Written by the indexing model from the issue text.