fix: changed modifier permanently suppressed requests on form elements

[MichaelWest22]

Description

Fix changed modifier on forms (closes #4033)

What was broken

hx-trigger="keydown changed" on a <form> silently suppresses every request. Doesn't matter what you type — nothing fires. Works fine on a bare <input>, so it's easy to miss until you try the natural pattern of triggering on the form itself.

Why

The changed block checked .value on the element carrying hx-trigger. For a form that's form.value, which is always undefined:

for (let fromElt of fromElts) {
    if (values.get(fromElt) !== fromElt.value) {  // undefined !== undefined — always false
        changed = true;
    }
}
if (!changed) return;  // always hits this

The fix

Track evt.target instead — the actual input the user typed in. That's always correct regardless of whether the listener is on the input directly, a parent form, or a from: element. Seed on first sight so we don't fire before anything has actually changed.

if (spec.changed) {
    let values = spec.values ??= new WeakMap();
    let target = evt.target;
    let value = target?.value;
    if (!values.has(target)) { values.set(target, value); return; }
    if (values.get(target) === value) return;
    values.set(target, value);
}

Three tests added: direct input suppression, the form bubbling case from #4033, and two inputs in one form tracked independently.

Corresponding issue:
#4033

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[1cg]

i think changed doesn't work w/ checkboxes & radio buttons as well. Just stumbled on that looking at:

#2896

[MichaelWest22]

Fixed checkboxes and changed support now. best to just ignore and skip them as changed makes no useful sense for them