Optimize Visitor::visitInParallel

[OpaqueRock]

Split out from #1948, so the Visitor optimization can be reviewed independently of the Lexer::readString optimization (which remains in #1948).

Problem

Visitor::visitInParallel() builds an enter/leave dispatcher closure that is invoked once per AST node. On every call, for every wrapped visitor, it called extractVisitFn() to re-resolve the enter/leave callback for that visitor and the current node's kind — an O(nodes × visitors) number of array lookups. However, a callback's identity for a given (visitor, kind, isLeaving) triple never changes during a single traversal, so this work was being needlessly repeated for every node of the same kind.

Additionally, the dispatcher closures used func_get_args() plus argument unpacking ($fn(...func_get_args())) even though they are always invoked with the same five arguments, which carries avoidable per-call overhead.

Change

• Cache the resolved enter/leave callback per visitor index and node kind, keyed lazily on first use (rather than prefilled from NodeKind's fixed set), using false as a "no callback" sentinel so ?? can distinguish "not yet cached" from "cached, nothing to call." Building the cache lazily also means visitors for custom (non-NodeKind) Node kinds are cached and dispatched correctly, same as before.
• Replace func_get_args() / argument unpacking in the enter/leave closures with the five explicit, typed parameters ($node, $key, $parent, array $path, array $ancestors) that match Visitor::visit()'s single call site.

No public API or behavior changes; this is purely an internal performance optimization of visitInParallel()'s dispatch path.

Testing

• composer test — full suite passes.
• composer stan — no new errors.
• composer rector / composer php-cs-fixer — no changes required.

[Copilot]

Pull request overview

Improves traversal performance in Visitor::visitInParallel() by avoiding repeated callback resolution and avoiding per-node func_get_args() overhead in the parallel visitor dispatch path.

Changes:

• Adds lazy per-(visitor index, node kind) caching for resolved enter/leave callbacks (with false sentinel for “no callback”).
• Replaces argument unpacking via func_get_args() with explicit dispatcher parameters to match the Visitor::visit() call site.

Comments suppressed due to low confidence (1)

src/Language/Visitor.php:452

• Same as enter: making leave require 5 parameters can break callers that invoke the dispatcher directly with only (Node $node). Consider defaulting the extra parameters to keep the callable usable in both 1-arg and 5-arg contexts.

            'leave' => static function (Node $node, $key, $parent, array $path, array $ancestors) use ($visitors, $skipping, $visitorsCount, &$leaveFns) {

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