Improve test coverage: untested branches

[darccio]

Current coverage is ~85.7%. The branches below were identified by running go test -coverprofile and inspecting uncovered statements. Each item is a distinct, reachable (or questionably reachable) code path with no test.

mergo.go — isEmptyValue

• case reflect.Func: return v.IsNil() (line 57) — covered by TestMergeFuncField added in the same scan session
• case reflect.Ptr/Interface + shouldDereference=false → return false (line 55–56) — non-nil pointer with WithoutDereference

merge.go — deepMerge

Error propagation (require a transformer that returns an error)

• Struct field loop: error returned from recursive deepMerge (line 93)
• Map value (struct/ptr/map kind): error from deepMerge (line 151)
• Map value (sliceDeepCopy loop): error from deepMerge (line 187)
• Top-level slice (sliceDeepCopy loop): error from deepMerge (line 245)
• Pointer dereference: error from deepMerge(dst.Elem(), src.Elem()) (line 267)

Slice handling

• AppendSlice type-mismatch for top-level slices — "cannot append two slice with different type" (line 230–232); only reachable by calling deepMerge directly, not via the public API (which rejects mismatched types earlier in resolveValues)
• sliceDeepCopy for slices stored as map values (line 174–192) — tested for struct fields but not map values

Map handling

• Re-initialise nil dst map before SetMapIndex when both src and dst enter as nil (line 207–209)

Pointer / interface handling

• dst.Elem().Type() == src.Type() branch: interface dst, non-pointer src of matching concrete type (line 275–278)
• ErrDifferentArgumentsTypes inside pointer/interface handling (line 279–281)
• Both dst and src are reflect.Interface, non-nil, no overwrite, same element kind (line 292–296)

mergo.go — resolveValues

• ErrNotSupported: pointer-to-primitive passed to _map (line 71–73)

map.go — _map

• ErrNonPointerArgument for Map() (line 144–146) — tested for Merge() but not Map()
• resolveValues error in _map: nil dst or src passed to Map() (line 157–159)
• ErrExpectedMapAsDestination: struct src, non-map dst (e.g. a slice) (line 167–169)
• ErrExpectedStructAsDestination: map src, non-struct dst (e.g. a slice) (line 171–173)
• ErrNotSupported in _map: src is neither struct nor map (line 174–175)

map.go — deepMap

• Empty-string guard in changeInitialCase (line 19–21) — no empty-string map keys in existing tests
• Cycle detection (line 41–44) — no self-referential struct tests
• !srcElement.IsValid() for nil map values (line 98–100)
• dstKind == Interface branch (line 105–108)
• srcKind == Map recursive deepMap branch for nested map→struct (line 109–112)
• Error from deepMerge in struct case (line 102–104)
• dstKind == Ptr && srcElement.CanAddr() (line 91–95) — likely unreachable; map values are never addressable in Go (the comment in the source says "Can this work? I guess it can't.")

Defensive / likely unreachable

• !srcElement.IsValid() inside map key iteration (line 120) — map keys always produce valid values
• !srcElement.CanInterface() for map elements (line 134) — map values are always interfaceable