test: characterize CLI behaviour

[shayyz-code]

Summary

This PR records how the poo binary behaves in v0.1.11, so the upcoming CLI refactor has to change it on purpose. It adds a cli module to the language_specs test binary (tests/language_specs/cli.rs) with 18 spec_cli_* specs. Each spec runs env!("CARGO_BIN_EXE_poo") in a fresh temp directory that is deleted afterwards. The interpreter and the conformance harness are unchanged.

Behaviour the specs record:

• A successful program exits 0 with an empty stderr. A top-level return adds Result: <Debug> to stdout, such as Result: Int(3) or Result: Vector([Int(1), Int(2)]). Without a return, and for an empty file, nothing extra is printed.
• Parse, lexer, runtime, and Io errors go to stderr as <Kind> error: in '<path>': <message> (Io: failed to read '<path>': ...) and exit 1. Every one of these specs also asserts that the exit code is not 101 (a panic).
• Output written before a runtime error stays on stdout.
• With no arguments, the binary prints Usage: <program> <file_path> to stderr and exits 1.
• --help and --version are read as file paths, which gives an Io error and exit 1. So is --speed when it comes before the file path.
• --speed after the path adds \nInterpreted within: <Duration Debug> to stdout. The spec checks the shape of that line, not the number. On an error the line is not printed.
• Extra unknown arguments are ignored without any message.
• spec_cli_runs_recursive_function is #[ignore = "#45: ..."]. Today the binary panics with exit 101 instead of printing Result: Int(120).
• spec_cli_reports_non_utf8_argument_without_panicking is unix-only and #[ignore = "#110: ..."]. Today a non-UTF-8 argument such as b"\xff.poo" panics in std::env::args() with exit 101; the spec expects exit 1 with an Io or usage error.

Assertions check stable substrings. They do not depend on the absolute temp path or on timing values.

Issue

Closes #90

Verification

• cargo test --test language_specs cli:::

  running 18 tests
  test cli::spec_cli_reports_non_utf8_argument_without_panicking ... ignored, #110: CLI panics with exit 101 on non-UTF-8 arguments
  test cli::spec_cli_runs_recursive_function ... ignored, #45: interpreter panics with exit 101 on recursive calls
  test cli::spec_cli_reports_missing_file_as_io_error ... ok
  test cli::spec_cli_prints_usage_without_arguments ... ok
  test cli::spec_cli_reports_parse_error_on_stderr_with_exit_code_1 ... ok
  test cli::spec_cli_speed_flag_appends_timing_line ... ok
  test cli::spec_cli_reports_directory_path_as_io_error ... ok
  test cli::spec_cli_ignores_extra_unknown_arguments ... ok
  test cli::spec_cli_keeps_output_printed_before_runtime_error ... ok
  test cli::spec_cli_runs_program_and_exits_zero ... ok
  test cli::spec_cli_reports_runtime_error_on_stderr_with_exit_code_1 ... ok
  test cli::spec_cli_treats_help_flag_as_file_path ... ok
  test cli::spec_cli_treats_version_flag_as_file_path ... ok
  test cli::spec_cli_speed_flag_omits_timing_line_on_error ... ok
  test cli::spec_cli_speed_flag_before_path_is_treated_as_file_path ... ok
  test cli::spec_cli_prints_top_level_return_value_in_debug_form ... ok
  test cli::spec_cli_prints_nothing_extra_without_top_level_return ... ok
  test cli::spec_cli_error_cases_exit_with_code_1_not_101 ... ok
  
  test result: ok. 16 passed; 0 failed; 2 ignored; 0 measured; 58 filtered out
  

• cargo test --test language_specs cli::spec_cli_reports_non_utf8 -- --ignored fails as expected: exit 101 from the env.rs unwrap.

• cargo test --test language_specs cli::spec_cli_runs_recursive_function -- --ignored fails as expected. The binary exits 101, and stderr shows the panic at src/type_inference.rs:117:36: Undefined function: fact.

• just check passes (fmt, check, clippy, tests including the conformance corpus, mdbook build and test, dist generate --check, cargo deny check).

• Each commit is a self-contained Conventional Commit and passes just check

Documentation and Visual Impact

None. Only tests changed. The module list in tests/language_specs/main.rs now includes cli.

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