The inspector never shows the instances field, so scaling out is unreachable #96

What happened

The inspector never shows the Instances field, so you can't scale a component out from the panel. Only Slots per instance can be changed.

What I did: started the Keep it warm challenge, clicked the Database, and looked for a way to add machines. The hint says "make what they land on able to take them". Expected an Instances (machines) field under "How much it can handle". There isn't one on any kind: service, db, cache, lb, worker and so on.

Cause: FIELDS_BY_KIND lists instances for most kinds, and FIELD_SPECS.instances defines it ("How many copies of this component are running. This is what an autoscaler adds and removes."). But the inspector only renders fields that belong to a FIELD_GROUPS entry (src/components/Inspector.tsx, the grouped filter), and instances isn't in any group. So it's dropped silently. The same filter feeds the multi-selection panel.

The engine and the canvas badge both honour instances, so the knob works. It just can't be reached.

Preset

Cache Aside (via the "Keep it warm" challenge). Reproduces on any preset or a freshly dropped component.

Load

Any. It's a render-time filter, independent of load.

Browser and OS

macOS. It doesn't depend on the browser, since the field is filtered out before render.

Anything else

Fix proposed in #95: add 'instances' to the "How much it can handle" group, with a regression test that fails on main.