Make find() search descendants only, as jQuery does

[jakejackson1]

find() treated the nodes already in the match set as candidates for their own selector.
jQuery's .find() searches descendants; filter() is what asks whether the nodes in hand match.

$xml = '<root><a id="a1"><a id="a2"/><b id="b1"/></a></root>';

qp($xml, '#a1')->find('a');   // before: a1, a2    after: a2
qp($xml, '#a1')->find('*');   // before: a1, a2, b1    after: a2, b1

Based on #72 — merge that first. This is not stylistic sequencing; see below.

The document element stays matchable

qp() seeds the match set with the document element, not the document node, so it stands in
for jQuery's $(document). Without an exception for it, qp($xml)->find('root') could never
match — and several long-standing tests, correctly, expect it to. So the self-match is kept for
exactly that one node:

qp($xml)->find('root');    // still 1
qp($xml)->find(':root');   // still 1

Arguably the deeper fix is to seed with the document itself, which would remove the exception
entirely. That is a much larger change and is not attempted here.

Why this needs #72

Before #72, the selector-filtered traversal methods (next(), siblings(), prevAll(), …) test
a candidate with QueryPath::with($node)->is($selector), and is() ran a find(). They were
relying on the self-match to ask "does this node match?" — the very question find() should not
be answering. Applying this change to main alone breaks nine of them:

testHtml5  testBefore  testSiblings  testHTML  testNext
testPrev   testNextAll  testPrevAll  testPrevUntil

Once #72 routes them through NodeMatcher, they stop caring. On top of #72 this change breaks
exactly one test.

The one test changed

DOMQueryTest::testBefore searched for the node it was standing on:

qp($file, 'unary')->before('<test/>')->find(':root > test ~ unary')

Rewritten to search from top(), which is the shape the next two assertions in that same test
already use. Its intent — that before() inserted <test/> as a preceding sibling — is unchanged
and still asserted.

Relationship to #50

#50 broadened the wildcard self-match from "the root element" to "any node", to make is() work
on an element itself under the old is(). #72 replaces that is(), and this PR removes the
self-match generally, so that hunk becomes unnecessary. Expect a conflict in
DOMTraverser::initialMatchOnElement() between #50 and this branch; resolve toward this one.
Two assertions in Issue49Test reach find() for a node in their own set and will need the same
treatment as testBefore.

Verification

• vendor/bin/phpunit — 351 tests, 1175 assertions, 0 failures (2 pre-existing create_function skips)
• composer run lint and composer run lint:min-php — clean
• New tests/QueryPath/FindDescendantOnlyTest.php (5 tests) pinning the behaviour, the document-element exception, and filter() as the replacement

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.69%. Comparing base (e84dc23) to head (2254927).

Additional details and impacted files

@@                     Coverage Diff                     @@
##             fix-selector-matching      #73      +/-   ##
===========================================================
- Coverage                    89.71%   89.69%   -0.03%     
- Complexity                    1363     1366       +3     
===========================================================
  Files                           27       27              
  Lines                         3064     3066       +2     
===========================================================
+ Hits                          2749     2750       +1     
- Misses                         315      316       +1     

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