Add a first-class correlation key to the process instance

The (name, value) index plus findInstancesByVariable() solves the immediate problem and gives a portable lookup. One alternative worth weighing for the longer term, likely cleaner than prefix-indexing a TEXT value column: a first-class correlation key (business key) on the ProcessInstance itself.

It would be an indexed scalar column on orchestra_instance, set at start(), holding the identity the process is about. Most engines model this explicitly (Camunda business key, Zeebe correlation key). Advantages for integrators:

• It is a real column on the instance, so a downstream entity can hold a native foreign key to the instance and follow it as a plain Views relationship: no value-scan, no JSON-string matching, no TEXT prefix index.
• It keeps "the identity this process is about" separate from process data (variables), which is what variables are for.

The variable lookup stays useful for ad-hoc correlation on arbitrary variables, so the two are complementary; the correlation key is the dedicated joinable identity. It could land as a follow-up rather than block this issue.

For context, on the integrator side (yoyaku) we are taking the equivalent step now: storing the instance id as an indexed reference on the transaction entity, so the reverse lookup and the dashboard relationship are a native FK rather than a variable match.