[sorvell]

Follow-up to #668 and #669. Base: scoped-registry-cross-realm (#669).

The result of several review passes over #668, with these goals: correct, robust, readable, simple to maintain, and small. The changes fall into two kinds.

Simpler, easier-to-maintain code

• Each rule lives in one place:
  • getRegistry, split into small helpers for the three ways a node gets its registry;
  • withAdoption for every adopting insertion;
  • one tree walk (forEachInTree);
  • one parsing wrapper (replaceChildrenByParsing).
• Dead code is gone: unused maps, fields and parameters, no-op guards, redundant checks.
• Comments are corrected or rewritten where stale or unclear, and declarations sit next to their first use.

Correct, robust behaviour
Where the polyfill differed from native, it now follows the spec:

• Which registry an element has: when it's created (including with new X()), cloned, adopted or moved, including when an insertion throws partway.
• When it's customized: at most once. A failed upgrade is final and clears the element's definition, as natively.
• What it gets: attribute callbacks in attribute order; form callbacks only for form-associated elements, including on upgrade; attachInternals only for custom elements; argument checking as in WebIDL.
• Missing interception points are added, e.g. Document.append, textContent and setHTML.

Soundness

• No API changes.
• Every behaviour fix has a test that failed before it. The full suite passes in Chromium, Firefox and WebKit, with and without force.
• WPT: 3 more tests pass in every browser, so each known-failure list shrinks by 3.
• Size: +0.5 KB minified, +0.3 KB gzipped.