Pre-epoch timestamps produce invalid millisecond fields

`DateUtil.createNormalDateTimeString` computes the millisecond fraction with truncating integer division. For timestamps before the Unix epoch this can produce a negative fraction.

For example, `DateUtil.createNormalDateTimeString(-1L)` currently returns `1969-12-31T23:59:59.0-1Z` instead of `1969-12-31T23:59:59.999Z`. The same path is used by `InstantAdapter.printDate`, so pre-1970 instants do not round-trip correctly.

The fraction should use floor-modulo semantics so it is always in the range 0-999. A regression test should cover both formatting and round-trip parsing.

Start with DateUtil.createNormalDateTimeString and trace its use from InstantAdapter.printDate. Reproduce the -1L example, then inspect the existing date-formatting and parsing tests to add coverage for pre-epoch formatting and round-trip parsing. Done means the millisecond field is always 0-999 and the regression tests pass.

Written by the indexing model from the issue text.