This change starts the host_env isolation plan by moving the shared OS-facing helpers out of rustpython-common into a dedicated rustpython-host-env crate. The goal is to make host access explicit in crate dependencies and reduce accidental leakage of host-only code into sandbox-oriented builds.

• New rustpython-host-env crate

  • Adds crates/host_env to the workspace
  • Introduces rustpython-host-env as the home for shared host abstractions with no Python runtime dependency
  • Exposes the migrated modules through a small crate root:
    • os
    • crt_fd
    • fileutils
    • fs
    • windows
    • macros

• Moved host-specific shared code out of rustpython-common

  • Relocates the existing shared host helpers from crates/common/src/ into crates/host_env/src/
  • Removes these modules from rustpython-common
  • Drops nix, windows-sys, and widestring from rustpython-common now that those dependencies belong to the new host crate

• Dependency graph updated

  • Adds rustpython-host-env to workspace dependencies
  • Wires both rustpython-vm and rustpython-stdlib to depend on rustpython-host-env
  • Re-exports the crate from rustpython-vm as vm::host_env for internal/external consumers

• Import migration in VM / stdlib / top-level consumers

  • Replaces direct uses of rustpython_common::{os, crt_fd, fileutils, windows, suppress_iph} with rustpython_host_env (or crate::host_env / vm::host_env where appropriate)
  • Updates representative host-facing call sites across:
    • VM stdlib modules
    • stdlib crate modules
    • top-level binary entrypoints
    • examples

• Expanded Phase 2 / Phase 4 extraction into rustpython-host-env

  • Moves additional self-contained host-facing layers into rustpython-host-env beyond the shared common move
  • Adds new host modules:
    • posix
    • signal
    • time
    • shm
    • select
    • syslog
    • termios
    • fcntl
    • msvcrt
    • nt
    • winapi
  • Rewires the corresponding VM / stdlib call sites to use those helpers from rustpython-host-env
  • Includes the wasm-target compatibility fix needed after the first partial extraction so the moved time helpers no longer break non-unix/non-windows builds

• Added Phase 5 lint enforcement for host access

  • Adds crate-local clippy.toml configuration for rustpython-common, rustpython-vm, and rustpython-stdlib
  • Enables #![deny(clippy::disallowed_methods)] in those three crates
  • Blocks new direct uses of selected std::fs, std::env, std::process, and std::net host APIs outside rustpython-host-env
  • Leaves rustpython-host-env exempt via its own local Clippy config

• Follow-up extraction and gating from review feedback

  • Replaces the reviewed temporary Phase 5 exemptions with actual extraction or host_env-only gating
  • Expands rustpython-host-env with generic helpers for:
    • filesystem access in a dedicated fs module (open, read, read_to_string, read_dir, create_dir_all, remove_dir, remove_file, metadata, symlink_metadata, open_write)
    • environment/process access in os (current_dir, temp_dir, var, var_os, vars_os, set_var, remove_var, set_current_dir, process_id, exit)
    • POSIX thread counting (posix::get_number_of_os_threads)
  • Keeps fileutils.rs focused on the fileutils.c counterpart surface while moving the generic filesystem helpers into the new crates/host_env/src/fs.rs
  • Rewires the reviewed VM / stdlib call sites to those helpers, including:
    • traceback source reads
    • file-running / pyc-file loading paths
    • sys.__breakpointhook__ environment lookup
    • readline history persistence
    • os / posix filesystem and environment helpers
    • SSL / OpenSSL certificate file and directory loading
    • faulthandler and posixsubprocess exit paths
  • Gates reviewed file-based VM APIs behind host_env where appropriate instead of leaving direct host calls available in non-host_env builds

• CI follow-up fixes tied to the extraction work

  • Fixes the wasm-oriented rustpython-common::refcount_overflow() path so it no longer relies on libc::abort() on wasm targets
  • Fixes the remaining Windows-specific current-directory access in vm::stdlib::os to use the extracted host_env wrapper

Example of the migration pattern:

// before
use rustpython_common::crt_fd;
let code = rustpython_vm::common::os::exit_code(exitcode);

// after
use rustpython_host_env::crt_fd;
let code = rustpython_vm::host_env::os::exit_code(exitcode);

This keeps the behavioral surface unchanged while making host-environment coupling explicit at the crate boundary, which is the foundation for tighter sandbox enforcement in follow-up work.