WPT results: count subtests gained and lost in each direction

[nicoburns]

Summary

The WPT results section in PR descriptions undercounted when subtests of one test moved in both directions. The headline summed each test's net pass-count change, and wpt diff (without --verbose) omits a test whose status and pass count are both unchanged. So a test that gains 40 subtests and loses 40 was missing from the headline and the full diff, while the by-area table (which compares subtests by name) counted it.

Example from #1046: headline "25 newly passing, 0 newly failing", by-area table +25 (+65 / -40). The difference is css/css-sizing/stretch/block-height-008.html, 80/160 before and after with 40 subtests fixed and 40 regressed.

Changes:

• wpt.yml: the JSON diff is generated with wpt diff --verbose, which joins subtests by name and reports tests whose individual subtests changed.
• wpt_diff_to_pr.py: gained/lost are counted per subtest from each entry's subtests list, falling back to the net change when the list is empty (tests without subtests, or a diff produced without --verbose).
• A test with changes in both directions is shown as ! with +G/-L instead of the net:

! FAIL => FAIL  [80/160]  +40/-40  css/css-sizing/stretch/block-height-008.html

• Entries where only non-passing subtest statuses changed (e.g. FAIL to TIMEOUT) with the same test status and pass count are left out, as before. --verbose would otherwise add them to the list.

wpt-post-results.yml runs the script from main against the artifact of the PR's run, so until a PR is rebased past this its diff has no subtest lists and renders as it does today.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/3a06d93089f2434ba3519a7813dea1e8
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/3a06d93089f2434ba3519a7813dea1e8?variant=devin-insiders
Requested by: @nicoburns

WPT results

No changes in test results compared to main.

_{Generated by the WPT workflow.}

[staging-devin-ai-integration]

I'll fix CI failures and address comments from users with write access that start with 'Devin'.

• Disable automatic comment, CI, and merge conflict monitoring