feat(require-input-label): add checkLabelFor option for sibling <label for> verification

[johanrd]

Summary

Adds an opt-in checkLabelFor option to template-require-input-label that verifies an <input id="X"> has a matching <label for="X"> in the same template — catching typos and forgotten labels that the existing rule treats as "probably labelled."

Builds on #2767. The diff is layered on top of that fix and is easiest to read against it. If preferred, review both PRs together here — the underlying bug fix is the first commit in this branch's history (already approved separately in #2767).

Why opt-in (default false)

Ember apps frequently split labels and form controls across component templates (design system wrappers). Enabling this check by default would false-positive on those patterns. Teams whose templates colocate label and input can opt in.

How it works

• Single visitor pass collects static <label for="X"> values into a Set
• Id-only controls (no aria-label/aria-labelledby/wrapping <label>) are deferred to Program:exit — so forward-declared labels (label after input) are captured
• Dynamic values (id={{this.fieldId}}, (unique-id) bindings, helper invocations) fall back to the existing skip behaviour — we deliberately don't resolve bindings symbolically
• Cross-reference is O(n + m) via Set lookup (n = label nodes, m = id-only controls)

Prior art

• angular-eslint label-has-associated-control has a checkIds: true opt-in that takes the same two-pass approach

Catches

{{! Typo in for= — silent today, flagged with checkLabelFor: true }}
<label for="emal">Email</label>
<input id="email" />

{{! Forgotten label — silent today, flagged with checkLabelFor: true }}
<input id="email" />

Doesn't false-positive on

{{! Common Ember (unique-id) pattern — both for/id are dynamic, skipped }}
{{#let (unique-id) as |myId|}}
  <label for={{myId}}>Name</label>
  <input id={{myId}} />
{{/let}}

{{! id with aria-label — id is irrelevant, validated via aria-label }}
<input id="email" aria-label="Email" />

Test plan

• 16 new tests covering: label-before-input, label-after-input (forward ref), id+aria-label not deferred, dynamic id/for fallback, unique-id pattern, labelTags+checkLabelFor combo, missing label, typo'd for
• All existing tests still pass (128 total, was 112)
• Docs updated with checkLabelFor explanation and false-positive caveat