[cli][doc] Add benchmarks against bc and python + Add shell completion guide in user manual

[forfudan]

This PR expands the CLI’s documentation and validation tooling by adding an end-to-end benchmark suite (vs bc and python3) and improving CLI file-mode test coverage using checked-in fixture files.

Changes:

• Add benches/cli/bench_cli.sh and wire it into pixi run bench cli.
• Expand tests/test_cli.sh file-mode coverage using new fixtures under tests/cli/test_data/.
• Document shell completion setup and publish performance/benchmark guidance in the user manual, plans, and changelog.

[Copilot]

Pull request overview

This PR expands the CLI’s documentation and validation tooling by adding an end-to-end benchmark suite (vs bc and python3) and improving CLI file-mode test coverage using checked-in fixture files.

Changes:

• Add benches/cli/bench_cli.sh and wire it into pixi run bench cli.
• Expand tests/test_cli.sh file-mode coverage using new fixtures under tests/cli/test_data/.
• Document shell completion setup and publish performance/benchmark guidance in the user manual, plans, and changelog.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test_cli.sh	Reworks file-mode tests to use committed fixture files instead of temp files; adds broader file-mode scenarios.
tests/cli/test_data/basic.dm	Adds a baseline file-mode fixture (constants + arithmetic).
tests/cli/test_data/comments.txt	Adds a fixture covering comments/blank lines/inline comments/whitespace handling.
tests/cli/test_data/edge_cases.dm	Adds a fixture for zero/negative/unary minus/parentheses/power edge cases.
tests/cli/test_data/precision.dm	Adds high-precision stress fixture (repeating decimals, cancellation, near-integer constants).
tests/cli/test_data/torture	Adds complex-expression fixture to stress tokenizer/parser/evaluator paths.
pixi.toml	Ensures CLI binary is built before running tests and testcli.
docs/user_manual_cli.md	Adds “Shell Completions” section and a “Performance” section referencing the benchmark suite.
docs/plans/cli_calculator.md	Marks performance validation and CLI documentation items as completed with references.
docs/changelog.md	Documents shell completion docs and the new CLI benchmark suite.
benches/run_bench.sh	Adds cli as a benchmark type and dispatches to the new shell benchmark script.
benches/cli/bench_cli.sh	Introduces correctness + timing comparisons vs bc and python3 across multiple expressions/precisions.

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