fix(core): order post watchers after render in flush

[shoemoney]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the main branch for v2.x (or to a previous version branch)
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

Other information:

When queueWatcher is called during a flush, the new watcher is spliced into the queue by id only. A post watcher (flush: 'post') can therefore land before a render watcher with a higher id, which breaks the guarantee that render runs before post callbacks.

This PR replaces the id-only comparison in the insertion loop with sortCompareFn, which orders non-post watchers before post watchers and then by id. That matches the initial queue sort, so post watchers queued mid-flush run after render watchers.

Before: queue[i].id > watcher.id
After: sortCompareFn(queue[i], watcher) > 0

Verified red to green. The existing unit suite passes (1449 tests, scheduler.spec 8 tests).

Changed file: src/core/observer/scheduler.ts