refactor(taint): deduplicate taint engine across JS/PY/Go

[Darkroom4364]

~8,500 lines of near-identical taint analysis spread across three files:

• `javascript_taint.rs`
• `python_taint.rs`
• `go_taint.rs`

`javascript_taint.rs` already notes it was "ported from python_taint.rs" and that "a future refactor is cheap."

Approach

• Extract shared traversal, propagation, and source/sink matching into a generic taint engine
• Language-specific files provide source/sink definitions and AST adapters
• Also addresses the 56 `.clone()` calls in `javascript_taint.rs` — many exist because ownership isn't structured for the shared traversal pattern