Summary

• handle addressable array-backed values through the existing common-interface unmarshalling path
• add a focused regression test for string slices bound to array-backed UnmarshalText values
• record the mandatory red-green bug-fix gate and the verified conversion behavior in OpenSpec

Test plan

• Before the production fix, go test -count=1 -run '^Test_Issue4786$' ./util/gconv failed with reflect: call of reflect.Value.IsNil on array Value
• After the fix, the same targeted command passed
• go test -count=1 -race ./util/gconv/...
• direct conversion into []uuid.UUID
• make lint
• openspec validate --all --strict

Fixes #4786