Support filter includes without side effects

[dloss]

Motivation

• Allow --include to be used with --filter so shared helper functions can be reused in filter expressions while preserving filter purity.
• Avoid introducing side effects into filter stages by preventing include files from contributing statements that could mutate state or perform I/O.
• Carry include metadata through the CLI/pipeline plumbing so filters can access included helper definitions.

Description

• Change ScriptStageType::Filter to carry { script: String, includes: Vec<IncludeFile> } and add IncludeFile { path, content } in src/config.rs.
• Add load_include_files in src/cli.rs and attach loaded includes to filter stages instead of inlining their contents into the expression.
• Add RhaiEngine::compile_filter_with_includes in src/engine.rs which compiles the filter as an expression, compiles each include as a script, rejects includes that contain statements, extracts only functions via clone_functions_only, and merges them into the filter AST.
• Update pipeline builders and FilterStage::new signature to accept includes, and add unit tests to validate that includes provide helper functions and that includes with statements are rejected.

Testing

• Ran cargo fmt --all and formatting completed successfully.
• Ran cargo clippy --all-targets --all-features -- -D warnings with no warnings treated as errors.
• Ran cargo test and the full test suite completed with all tests passing (automated unit and integration tests passed).

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Codex Task