Migrate to new Zig compiler

[lukewilliamboswell]

Summary

• Migrate basic-cli to the new Zig compiler and flat platform host ABI.
• Consolidate the host into the Rust roc_host static library and restore the public CLI platform surface for commands, files and directories, environment access, HTTP, SQLite, TCP, terminal I/O, time, and related utilities.
• Build platform inputs for macOS Apple Silicon and Intel, Linux musl x64 and ARM64, and Windows x64.
• Replace the legacy shell/Expect suite with a cross-platform, spec-driven runner based on 29 executable examples and 41 runtime cases.
• Adopt the shared release and versioned-docs actions from roc-lang/release-package.

Public API and migration notes

This is a breaking platform release, not only a compiler implementation change.

• Application arguments are native OsStr values, preserving Unix bytes and Windows UTF-16. The old Arg module is replaced by OsStr.
• Paths use the shared roc-lang/path package types, and HTTP requests/responses use the shared roc-lang/http package types.
• Platform effects use the new Try/open-tag error style and expose structured IOErr values.
• Environment support includes native variable access, dict!, platform!, working-directory access/update, executable path, and temporary directory.
• The checked-in examples are executable documentation and the source of truth for cross-target build and runtime coverage.

Automated validation

The last complete GitHub matrix is green, including:

• formatting, checking, and testing all 29 examples;
• building every example for x64mac, arm64mac, x64musl, arm64musl, and x64win;
• running 41 spec cases natively on macOS Intel, macOS Apple Silicon, Linux x64, and Windows x64;
• assembling the complete .tar.zst platform bundle and consuming the downloaded bundle on those four native runners;
• generating and validating versioned documentation; and
• CodeQL analysis.

Runs: CI · release dry run

On the latest head, local validation with Roc debug-a06fe3fc passes formatting and checking for every example, all 66 Roc tests, git diff --check, Rust formatting and a locked all-target Cargo test-profile build, ShellCheck/Bash syntax, Python syntax, and test-spec JSON parsing.

The fresh GitHub run currently stops at roc fmt --check because the published release-fast-252a59ae nightly predates the formatter used for the final cleanup. It reaches the first changed SQLite example without a compile or test failure. Rerun the matrix after debug-a06fe3fc (or its formatter change) is published in the next nightly.

CI and release intentionally follow the current nightly-new-compiler alias. Zig is pinned to 0.16.0 and Rust to 1.82.0.

Release flow

The Release workflow is available through workflow_dispatch with a stable or release-candidate version. It:

1. validates the request and default branch;
2. builds and tests the release bundle on the supported native runners;
3. publishes the release and marks prerelease versions automatically;
4. publishes validated versioned docs; and
5. opens a follow-up PR that updates example bundle URLs and commits the generated docs tree.

The shared release actions are pinned to d2560ead9f724bfaabfbc8134b8895e120171064.

bump_check is temporarily warn: stable 0.20.0 uses the old .tar.br format and Roc syntax, while alpha-0 no longer parses with the current compiler. Change this to require after the first stable new-compiler .tar.zst release.

Known limitations and coverage notes

• The Windows time/sleep runtime currently crashes, so examples/time.roc builds on Windows but its runtime case is skipped. This is accepted for the release candidate.
• ARM64 Linux is compile-only because no matching runner is available.
• The command example's runtime case invokes Unix commands.
• Terminal and TTY runtime cases use POSIX PTYs; the Python harness does not provide Windows ConPTY support.
• Windows file-permission metadata returns the documented Unsupported result.
• Nix checks remain temporarily removed.

Final review

The branch is based on the current main and is mergeable. This PR remains a draft while the final maintainer review is completed; it can be marked ready once that review and the fresh GitHub matrix are satisfactory.