fix(time): precisedelta keeps the sign of negative timedeltas

[Mukller]

Fixes #379

precisedelta() silently dropped the sign of negative timedeltas: -3661 s
rendered byte-identical to +3661 s, making the function unsafe for signed
inputs.

Root cause

The sign was lost before precisedelta() even ran: the shared helper
_date_and_delta() returns _abs_timedelta(delta) (time.py), so every caller
receives the absolute value. precisedelta then decomposed already-positive
components.

Changes

• precisedelta() captures the sign from the original value (timedelta
  comparison, datetime-vs-now comparison, or numeric < 0) and prefixes the
  rendered string with - for negative inputs. Zero stays unsigned.
• Both return paths covered (single-unit and joined multi-unit).

Testing

Four new tests in tests/test_time.py: negative multi-unit, negative with a
coarser minimum_unit (float path), zero-stays-unsigned, positive-unchanged.
Full suite: 704 passed, 74 skipped.

Note: naturaldelta() shares the same helper, so it presumably also drops the
sign — left out of scope here to keep this PR focused, but worth a separate
issue if maintainers agree it's undesired there too.