feat: support new runtimes in ir crate

[ChefBingbong]

Auto-generated PR from staged changes.

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Note

Medium Risk
Behavior changes from always parsing imports for supported languages to requiring runtime registration; if registration is missed or incomplete, generated code may silently lose import extraction.

Overview
Moves setup-block import parsing out of poly-bench-ir into a new poly-bench-ir-traits interface, and switches extract_imports to use a runtime-registered extractor registry (set_import_extractors) instead of built-in Go/TS/Rust/Python parsing.

Removes the in-crate language-specific extractors and their tests, updates re-exports accordingly, and adds poly-bench-ir-traits as a dependency; if no extractors are registered, import extraction now falls back to passthrough (no imports extracted).

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Import extractors registry is never initialized

High Severity

The EXTRACTORS static is never populated because init_import_extractors() (defined in poly-bench-runtime/src/registry.rs) is exported but never called anywhere in the codebase. This means extract_imports always takes the None branch and returns ParsedSetup::passthrough(setup) — silently discarding all language-specific import extraction (Go, TypeScript, Rust, Python) that previously worked via hardcoded extractors. Every call to ir::lower (four sites in cli/main.rs) will produce IR with empty imports.