Improper Control of Generation of Code ('Code Injection')

Summary

`piscina`'s constructor and `run()` paths read the `filename` option via plain member access. Both reads fall through the prototype chain when the caller's options object doesn't have filename as an own property. When Object.prototype.filename is polluted upstream -- by any of the well-documented PP-source CVEs (lodash<4.17.13, qs<6.10.3, set-value<4.1.0, minimist<1.2.6, deepmerge<4.2.2, and others) -- the inherited value flows to worker_threads.Worker import and the attacker's .mjs runs in the worker. Subtlety: calling pool.run(task) with no second arg uses kDefaultRunOptions which has filename: null as an OWN property -- that path DOES NOT fire. The vulnerable shape is when the caller passes their own options object (commonly {signal: ac.signal} for abort support, {name: ...} for task labelling, etc.). These caller-built options objects inherit from Object.prototype unless the caller explicitly uses Object.create(null). This issue affects prior to 4.9.3, 5.0.0-alpha.0 prior to 5.2.0 and 6.0.0-rc.1 prior to 6.0.0-rc.2.