Add runtime process I/O foundation (for cmd/shell MVP)

[synapticvoid]

Summary

Add a minimal runtime I/O/process layer so command execution can be implemented as fallible builtins and composed in scripts.

Why

#19 (shell call syntax) needs a stable execution substrate first.
Today runtime context is stdout-only and there is no process execution primitive.

Scope (MVP)

• Extend runtime context to support both stdout and stderr writers
• Add internal process execution primitive:
  • input: program + argv
  • output: { code: Int, stdout: String, stderr: String }
  • fallible on spawn/wait failures
• Define minimal error enum for runtime I/O/process failures:
  • e.g. IoError { SpawnFailed, WaitFailed, NotFound, PermissionDenied }
• Expose builtins:
  • cmd(spec) -> IoError!RunResult
  • shell(script: String) -> IoError!RunResult (explicit unsafe shell mode)

Out of Scope

• Async/background jobs
• Streaming stdout/stderr
• Env/cwd overrides per command
• Pipeline DSL (pipe/run/check) sugar
• File read/write builtins

Acceptance Criteria

• Running a known command returns exit code/stdout/stderr
• Unknown command returns an IoError variant
• Non-zero process exit is represented in RunResult.code (not a runtime crash)
• cmd and shell are both type-checkable as fallible functions
• Basic tests cover success, failure-to-spawn, and non-zero exit