[eval] Move expression evaluator from CLI to Decimo core library

[forfudan]

This PR promotes the CLI calculator’s arithmetic expression implementation into the Decimo core library as decimo.expression, exposing a stable high-level API (decimo.eval / decimo.evaluate) while updating the CLI and tests to consume the shared engine.

[Copilot]

Pull request overview

This PR promotes the CLI calculator’s arithmetic expression implementation into the Decimo core library as decimo.expression, exposing a stable high-level API (decimo.eval / decimo.evaluate) while updating the CLI and tests to consume the shared engine.

Changes:

• Introduces src/decimo/expression/ as a first-class core module (tokenizer, shunting-yard parser, RPN evaluator) and exports decimo.eval / decimo.evaluate.
• Updates the CLI calculator to reuse decimo.expression instead of maintaining its own copy.
• Adds/updates expression test suite wiring and documents the new API in the changelog.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test.sh	Adds an expression test suite and includes it in the core (decimo) test group and suite listing.
tests/expression/test_tokenizer.mojo	Updates imports to use decimo.expression for tokenizer symbols.
tests/expression/test_parser.mojo	Updates imports to use decimo.expression and consolidates parser/tokenizer imports.
tests/expression/test_evaluator.mojo	Updates evaluator tests to use decimo.expression APIs (evaluate/tokenize/parse/evaluate_rpn/final_round).
tests/expression/test_error_handling.mojo	Updates imports to call into decimo.expression for error-path tests.
src/decimo/prelude.mojo	Adds import decimo / import decimo as dm to the prelude exports.
src/decimo/expression/tokenizer.mojo	Adjusts module wording/imports for core usage and expands whitespace handling to include \n/\r; adds variable-awareness via optional known-variable map.
src/decimo/expression/parser.mojo	Updates module wording to reflect core expression engine usage.
src/decimo/expression/evaluator.mojo	Adds eval(...) as the high-level entry point with optional variables; keeps evaluate(...) as a backwards-compatible alias; updates imports to core-relative modules.
src/decimo/expression/init.mojo	New public module surface for decimo.expression, re-exporting tokenizer/parser/evaluator APIs and constants.
src/decimo/init.mojo	Exports eval and evaluate from decimo.expression at the top level.
src/cli/calculator/repl.mojo	Switches REPL tokenizer helpers to import from decimo.expression.
src/cli/calculator/engine.mojo	Switches engine pipeline (tokenize/parse/evaluate/final_round) to use decimo.expression.
src/cli/calculator/init.mojo	Re-exports expression engine symbols from decimo.expression for CLI package consumers.
docs/changelog.md	Documents the new decimo.expression module, decimo.eval, variables support, and newline/CR whitespace behavior.

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