ci: pin spec-prod to v2.12.5

[marcoscaceres]

The w3c/spec-prod@v2 floating tag (currently v2.13.0, released 2026-03-04) fails on every PR and on gh-pages push runs with:

Command `respec -s "..." -o "index.html.built.html" --verbose -t 20 -e` failed with exit code: 127.

Exit 127 = respec not found on PATH. Reproduced on:

• This repo's recent gh-pages push run (21970293086, 2026-02-13)
• Two unrelated PR runs (21970234675, 24270554650)
• A fresh re-run today (25891886076)

Local npx respec --localhost --haltonerror -s index.html -o /dev/null passes cleanly (exit 0, 0 errors), so the spec content is fine — the regression is in the action.

v2.12.5 (2026-02-26) is the last release before v2.13.0; pinning to it should restore CI while we wait for upstream to fix the bare-respec invocation. No upstream issue is filed yet — happy to file one once we confirm the pin works.

The following tasks have been completed:

• Modified Web platform tests — CI workflow change only, no spec or observable behavior change.

Implementation commitment:

• WebKit — n/a, no observable change
• Chromium — n/a, no observable change
• Gecko — n/a, no observable change

[marcoscaceres]

Pinning to v2.12.5 reproduces the same failure (run https://github.com/w3c/gamepad/actions/runs/25893348176) — the bug pre-dates that release. Filed upstream as w3c/spec-prod#229. Closing this PR until upstream is fixed.