fix: apply sign of chronometer and time strings to whole Duration

Duration::fromChronoString() and Duration::fromTimeString() only negated the
hour component of a signed input. When the hour was zero or absent the sign was
silently dropped, and when it was present the minute/second/fraction components
stayed positive, producing a duration that did not match the input. Prefix every
unit token with the sign so the whole duration is negated consistently.

PR.md

@@ -0,0 +1,104 @@
+# fix: apply the sign of chronometer/time strings to the whole Duration
+
+## Problem
+
+`Duration::fromChronoString()` and `Duration::fromTimeString()` accept an optional
+leading sign so callers can build negative durations (the grammar defines a
+`(?<sign>\+|-)?` group). The sign was mishandled in two ways:
+
+1. When the hour component is zero or absent, the sign is silently lost and a
+   positive duration is returned.
+2. When the hour component is present, only the hour is negated while the minute,
+   second and fraction components stay positive, producing a duration with mixed
+   sign components that does not represent the input.
+
+Examples on the current code:
+
+```
+Duration::fromChronoString('-05:30');   // minute:second -> +5m30s  (sign lost)
+Duration::fromChronoString('-1:05:30'); // -1h +5m +30s = -3270s, not -3930s
+Duration::fromTimeString('-00:30');     // -> +30m       (sign lost)
+Duration::fromTimeString('-01:30');     // -1h +30m = -1800s, not -5400s
+```
+
+Applying these durations to a date point therefore moves the date in the wrong
+direction or by the wrong amount.
+
+## Root cause
+
+`src/Duration.php`, `Duration::fromUnits()`. The negative sign was concatenated
+onto the hour string only:
+
+```php
+if ('-' === $units['sign']) {
+    $units['hour'] = '-'.$units['hour'];
+}
+```
+
+Because the string handed to `DateInterval::createFromDateString()` only carried a
+sign on the `hours` token, `-0 hours 30 minutes` normalized to a positive
+30-minute interval, and `-1 hours 5 minutes 30 seconds` kept the minute and second
+tokens positive.
+
+## The fix
+
+Prefix every unit token with the sign so the whole duration is negated
+consistently:
+
+```php
+$sign = '-' === $units['sign'] ? '-' : '';
+
+return self::fromDateString(
+    $sign.$units['hour'].' hours '
+    .$sign.$units['minute'].' minutes '
+    .$sign.$units['second'].' seconds '
+    .$sign.$units['fraction'].' microseconds'
+);
+```
+
+`DateInterval::createFromDateString()` accepts a per-token sign, so this produces
+a `DateInterval` whose components all carry the correct sign. Verified against a
+reference date:
+
+```
+2021-01-01 00:10:00 + fromChronoString('-05:30')   = 2021-01-01 00:04:30
+2021-01-01 02:00:00 + fromChronoString('-1:05:30')  = 2021-01-01 00:54:30
+2021-01-01 00:20:00 + fromChronoString('-12:28.5')  = 2021-01-01 00:07:31.500000
+2021-01-01 01:00:00 + fromTimeString('-00:30')      = 2021-01-01 00:30:00
+2021-01-01 05:00:00 + fromTimeString('-01:30')      = 2021-01-01 03:30:00
+```
+
+## Tests
+
+New data provider `signedDurationProvider` with the test
+`testSignedStringsApplySignToWholeDuration` asserts the real elapsed effect of the
+signed strings by applying them to a reference `DateTimeImmutable` (helper
+independent, covers the zero-hour, full h:m:s and fractional-second cases).
+
+Two existing data-provider rows pinned the buggy output and encode the defect, so
+they were corrected:
+
+- `fromChronoProvider` `negative chrono`: `-12:28.5 => PT12M28.5S` (sign dropped)
+  updated to a fully negative case `-12:28 => PT-12M-28S`.
+- `fromTimeStringProvider` `negative chrono`: `-12:28 => PT-12H28M` (mixed sign)
+  updated to `-12:28 => PT-12H-28M`.
+- `fromTimeStringProvider` `negative chrono with seconds`: `-00:00:28.5 => PT28.5S`
+  (sign dropped) updated to `-01:02:28 => PT-1H-2M-28S`.
+
+The fractional-second and seconds-only negative cases are covered by the new
+date-arithmetic test instead of the string helper, because that helper renders
+negative fractional seconds and bare seconds-only intervals with pre-existing
+quirks that are out of scope here.
+
+## Gate
+
+```
+composer install && vendor/bin/phpunit --no-coverage
+```
+
+Result: exit code `0` - `OK (562 tests, 933 assertions)`.
+
+Counterfactual: reverting `fromUnits()` to the hour-only sign makes all five
+`testSignedStringsApplySignToWholeDuration` cases fail, e.g.
+`fromTimeString('-01:30')` yields `2021-01-01 04:30:00` instead of the expected
+`2021-01-01 03:30:00`.