Solutions and pH is still in progress

Start with concentration in one beaker, add solubility limits and saturation, then reuse that same solution language to read pH, buffers, and neutralization.

This completion page stays public, but the completion reflection only unlocks after every concept and authored checkpoint in this track is clear on this browser. The route still reads the same local concept records and solved challenge ids, and signed-in sync keeps reusing those records instead of inventing a separate track store.