fix(decode): handle unaddressable values in interface decode

[xe-nvdk]

Summary

• Fix panic "reflect.Value.SetInt using unaddressable value" when decoding into an interface{} that already holds a non-pointer value type (e.g. int8)
• decodeInterfaceValue() now checks if the inner value is a pointer before attempting in-place decode; falls back to fresh-value decode for value types
• This is the canonical fix for 5 upstream issues (Decode panics on nil interface with pre-populated typed pointer (upstream #304) #21, SetString panic with empty struct pointer (upstream #339) #29, SetString panic with variadic marshal (upstream #336) #30, SetString panic (upstream #328) #31, RawMessage decode panic: unaddressable value (upstream #310) #32)

Closes #21, closes #29, closes #30, closes #31, closes #32.

Test plan

• go test -run TestTypes -count=10 — passes all 10 (previously panicked at count=5)
• go test ./... -v -count=1 — full suite passes
• go test ./... -short -race — clean
• go test -run='^$' -bench=. -benchmem -count=3 — zero regression