fix(compose): copy map values before assigning nested fields

[serhiizghama]

When two field mappings target sub-fields of the same map value (e.g. a.X and a.Y for a map[string]struct{...}), the second one panics with reflect.Value.Set using unaddressable value.

The first mapping creates the value through newInstanceByType (addressable) and stores it. The second mapping reads it back with MapIndex, which is never addressable, and then tries to Set a field on it. assignOne only handled the missing-key case, not the read-back case.

Fixed by copying the read-back value into an addressable value before descending into it; the existing SetMapIndex write-back at the end of the walk puts the updated struct back into the map, so the earlier field survives.

Added a test that maps two sub-fields into one map value — it panics on the current code and passes with the fix. go test ./compose/ is green.

[CLAassistant]

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