Make :enabled the complement of :disabled, fieldsets included

[jdalton]

Fix

Use one disabled-state predicate for :disabled, :enabled, :read-only, and :read-write. A disabled fieldset exempts only descendants of its first legend. Nested fieldsets cannot erase an outer restriction. Options inherit disabled state from their immediate optgroup, not their select or fieldset.

Tests

Focused regressions cover first and later legends, nested fieldsets, own disabled state, non-controls, option groups, and DOM changes. The complete local Docker CI workflow passes, including Node tests, package checks, Chromium, WPT on both builds, and unchanged coverage thresholds.

[jdalton]

This one sits with #191 and #192 and #193, which touch the conformance fixes found by comparing against Chromium.

They do not depend on each other. All seventeen in the series cherry-pick onto master in any order, and I checked that in both directions, so any one of these can land alone. The order below is the one they read best in:

• #190 makes :enabled the complement of :disabled, fieldsets included.
• #191 anchors the :required and :optional tests, and lets :optional take a button.
• #192 makes a fieldset :valid when none of its controls is invalid.
• #193 makes every built-in element :defined.