Implements row-decoding for IDS INTERVAL, the last common temporal type.
The qualifier short bisects the type at the wire level: start_TU >= DAY
maps to datetime.timedelta (day-fraction), start_TU <= MONTH maps to a
new informix_db.IntervalYM (year-month).
Wire format mirrors DATETIME exactly — `[head byte][digit pairs in
base-100]`, with the qualifier dictating field interpretation. The
fraction-to-nanoseconds scaling (`scale_exp = 18 - end_TU`, forced odd)
is the JDBC pattern from `Decimal.fromIfxToArray`.
IntervalYM is a frozen dataclass holding signed total months, with
`years` and `remainder_months` as derived properties. Matches JDBC's
`IntervalYM.months` shape rather than a (years, months) tuple — avoids
ambiguity around what "negative" means for a multi-field tuple.
Tests: 13 unit (synthetic byte streams covering all decoder branches)
+ 9 integration (real Informix queries spanning DAY TO SECOND, HOUR TO
SECOND, YEAR TO MONTH, negatives, NULL, and mixed-family rows).
Total test count: 53 unit + 82 integration = 135.
Encoder for INTERVAL parameter binding is deferred to a later phase
(same arc as DECIMAL/DATETIME — decode lands first).
Ryan Malloy
4dafbf8ce9