Add a neurotransmitter preset to the Neurochemistry Sandbox

[slavazeph-coder]

Layer 30 — brainsnn-r3f-app/src/utils/neurochemistry.js

Add a named preset (e.g. ketamineMicrodose, propofol, melatonin) with a 6-NT signature. Presets are objects of the form { dopamine, serotonin, cortisol, oxytocin, norepinephrine, acetylcholine } on a 0–1 scale.

There are 9 existing presets (caffeine, meditation, acute stress, SSRI 4 wk, MDMA phase II, …) to copy the shape from. Bonus points for citing the pharmacology you based the signature on in the PR description.