fix: subset false-positive with a prerelease eq and a differing bound

[spokodev]

What

subset(sub, dom) returns true (claims sub ⊆ dom) when it is actually false. It triggers when sub combines an exact prerelease comparator (=X.Y.Z-pre) with a >/>=/</<= bound whose version has a different [major, minor, patch] tuple — a subset false-positive (the dangerous direction):

semver.subset('=1.1.2-alpha <3.1.0', '<1.0.0') // true, must be false

// proof: a version in sub is not in dom, so sub can't be a subset of dom
semver.satisfies('1.1.2-alpha', '=1.1.2-alpha <3.1.0') // true  (in sub)
semver.satisfies('1.1.2-alpha', '<1.0.0')              // false (not in dom)

semver.subset('<3.1.0-0 1.1.2-alpha', '~2.0')  // true, must be false

Adding the <3.1.0 bound (which 1.1.2-alpha satisfies) can only shrink sub, so it should never turn a non-subset into a subset — a monotonicity violation.

Root cause

In simpleSubset, the eqSet-vs-bound checks used satisfies(eq, String(gt), options). satisfies builds a full Range and re-applies node-semver's prerelease-exclusion gating, so a prerelease eq (1.1.2-alpha) is judged not to satisfy a plain bound of a different tuple (<3.1.0). The code then treats the eqSet as inconsistent with sub and returns null (a null set), which subset reports as a subset of everything. In reality the eq version does satisfy sub, because its own =eq comparator admits its prerelease.

This is the same "re-applying full-range prerelease gating on an isolated comparator" issue that PR #867 fixed on the dom side (subset.js:177, :195), but it was left in place on the sub (eqSet) side (:127, :131).

Fix

Test the eq version against the raw bound comparator (gt.test(eq) / lt.test(eq)), mirroring PR #867.

Tests

Added two subset cases (=1.1.2-alpha <3.1.0 ⊄ <1.0.0, <3.1.0-0 1.1.2-alpha ⊄ ~2.0). Both fail on main and pass with the fix; issue #757's case (^10.2.0-beta.2 ⊂ ^10.2.0-beta.1) stays correct and the ranges tests pass.