Fix Intervals::isSubsetOf() returning false for < 0.0.0

[orcnd]

What

Intervals::isSubsetOf($x, $x) returned false for $x = < 0.0.0. Since < 0.0.0 matches nothing, it should be a subset of any constraint, like the existing < dev-foo case.

Why

• < 0.0.0 normalizes to < 0.0.0.0-dev, and generateSingleConstraintIntervals() turned it into the invalid interval >= 0.0.0.0-dev - < 0.0.0.0-dev.
• The multi-constraint path in generateIntervals() already filters out such >= x - < x intervals.
• So in isSubsetOf(), the intersection had 0 numeric intervals while the candidate had 1, and the count comparison failed.

How

For a < constraint on the lowest version (the version of Interval::fromZero()), return an empty numeric interval list with no dev branches, instead of the invalid interval. This follows:

• the documented get() convention: "if the returned numeric array is empty it means the constraint matches nothing in the numeric range";
• the >= x - < x check already used by the multi-constraint path.

An alternative would be to handle this only inside isSubsetOf(), but get() would then still return an interval that can never match. I fixed it where the interval is generated. Happy to switch if you prefer.

How tested

• SubsetsTest: < 0.0.0 is a subset of < 0.0.0 and of ^1.0.
• IntervalsTest: get('< 0.0.0') returns no numeric interval.
• These cases fail before the change and pass after.
• The full suite (2297 tests) and phpstan pass on PHP 8.2 and 8.5.
• haveIntersections() and compactConstraint() return the same results for this input as before.

Fixes #191

AI disclosure: this change was prepared with the help of Claude Code and reviewed by me.

🤖 Generated with Claude Code

[Seldaek]

Thanks!