Add decimal, datetime, date, and time type support across all UDF paths

Implement full serialization/deserialization for DECIMAL, DATETIME, DATE,
and TIME types in the C accelerator (accel.c), pure-Python rowdat_1 codec,
ASGI handler, and collocated registry. Previously these types raised
"unsupported" errors or were stubbed with TODO comments.

Key changes:
- accel.c: Implement load/dump for DECIMAL (length-prefixed UTF-8 string),
  DATE (packed YYYYMMDD int64), TIME (signed HHMMSSuuuuuu int64), and
  DATETIME/TIMESTAMP (bit-packed int64) in all three code paths
  (load_rowdat_1_numpy, load_rowdat_1, call_function_accel). Add interned
  PyStr attribute strings for datetime component access.
- rowdat_1.py: Add pack/unpack helpers and wire them into _load, _load_vectors,
  _dump, and _dump_vectors for the four new type families.
- asgi.py, collocated/registry.py: Add datetime/date/time/decimal entries to
  rowdat_1_type_map so the ASGI and collocated paths can negotiate these types.
- registry.py: Add _dtype_to_python mappings, emit datetime/decimal imports in
  generated code, convert silent skip-on-bad-dtype to TypeError for visibility.
- signature.py: Add 'decimal' to sql_type_map, DECIMAL/NUMERIC to
  sql_to_type_map, and normalize_dtype() mappings for decimal.Decimal and
  datetime.time.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kesmit13