fix: handle subset check across union of comparator sets

[deepakganesh78]

Fixes #703

Problem

subset('>=17.2.0', '^17.2.0 || >17') returns false but should return true.

The existing subset() implementation checks each dom comparator set independently via simpleSubset(). If no single dom set covers the sub range, it returns false — even when the union of all dom sets fully covers the sub range.

For this case:

• ^17.2.0 expands to >=17.2.0 <18.0.0-0 — covers [17.2.0, 18.0.0-0)
• >17 expands to >=18.0.0 — covers [18.0.0, +∞)
• Their union covers [17.2.0, +∞) which fully contains >=17.2.0

Root cause

simpleSubset() only checks sub against each dom set one at a time. There was no mechanism to consider the union of multiple dom sets.

Fix

Added unionCovers() as a fallback when dom has multiple OR-branches and no single branch covers the sub range. The algorithm:

1. Extracts interval bounds (gt/lt comparators) from each dom set
2. Filters out null-set branches (impossible ranges)
3. Sorts intervals by lower bound
4. Sweeps left-to-right, greedily extending coverage
5. Checks if contiguous coverage spans the entire sub range

Key details:

• Prerelease adjacency: In non-prerelease mode, <18.0.0-0 and >=18.0.0 are adjacent (no release version between them), so the algorithm recognizes this as contiguous coverage
• Bound comparison: Correctly handles inclusive (>=/<=) vs exclusive (>, <) bounds
• includePrerelease option: Respects the option by disabling prerelease adjacency shortcuts when enabled

Compatibility

This is a non-breaking change — it only makes subset() return true in cases where it previously (incorrectly) returned false. No existing behavior is changed.

Validation

• All existing tests pass
• 55+ new test cases added covering: multi-branch unions, gaps, overlapping intervals, prerelease adjacency, null-set branches, * ranges, inclusive/exclusive bounds, includePrerelease mode
• 100% code coverage (statements, branches, lines, functions)
• ESLint passes