fix: split dotted prerelease identifiers in inc()

[MerlijnW70]

Summary

SemVer.prototype.inc('prerelease', identifier) stores the identifier as a single element of the prerelease array even when it contains dots. A dotted identifier such as 'x.y' is stored as ['x.y', 0] instead of ['x', 'y', 0]. The formatted version string is correct, but the in-memory prerelease array is malformed, so comparePre() compares a dotted element against a plain identifier and mis-orders versions.

Reproduction

const { SemVer } = require('semver')
const semver = require('semver')

const a = new SemVer('1.2.3-alpha').inc('prerelease', 'x.y')
a.version            // '1.2.3-x.y.0'   (correct)
a.prerelease         // ['x.y', 0]      <-- should be ['x', 'y', 0]

a.compare(new SemVer('1.2.3-x.y.1'))          //  1   (claims x.y.0 > x.y.1)
semver.compare('1.2.3-x.y.0', '1.2.3-x.y.1')  // -1   (correct)

The same version compared as a SemVer object versus as its string yields opposite results. Because comparePre() walks the array element-by-element, the malformed 'x.y' element is compared with compareIdentifiers('x.y', 'x') and wins lexically, so 1.2.3-x.y.0 is reported as greater than 1.2.3-x.y.1.

Root cause

In classes/semver.js, the pre case of inc() assigns the raw identifier straight into the array:

let prerelease = [identifier, base]   // identifier may be 'x.y'
if (identifierBase === false) {
  prerelease = [identifier]
}

The surrounding code already knows identifiers can be dotted — a few lines down it uses identifier.split('.').length — but the assignment itself never splits. Every other consumer (comparePre, and the constructor) assumes one dot-free identifier per array element.

Fix

Split the identifier on . and numberify numeric ids, exactly as the SemVer constructor does when parsing prerelease identifiers, so comparePre() sees one identifier per element:

const identifiers = identifier.split('.').map((id) => {
  if (/^[0-9]+$/.test(id)) {
    const num = +id
    if (num >= 0 && num < MAX_SAFE_INTEGER) {
      return num
    }
  }
  return id
})
let prerelease = [...identifiers, base]
if (identifierBase === false) {
  prerelease = identifiers
}

After the fix, inc('1.2.3','prerelease','1.2').prerelease deep-equals parse('1.2.4-1.2.0').prerelease ([1, 2, 0]), and object comparison agrees with string comparison. Non-dotted identifiers ('beta', 'dev', …) are unaffected.

Tests

• Adds a dedicated regression test in test/functions/inc.js asserting the prerelease array shape and that object comparison matches string comparison (the fixture suite only checks the version string, which is why this went unnoticed).
• Adds fixture cases in test/fixtures/increments.js for dotted identifiers, including a numeric id and a numeric id ≥ 2^53 (kept as a string, matching the constructor).

Full suite passes with 100% coverage and lint clean.