Per-file batch fixing for --fix option

[mabar]

Each file's fixable errors are now applied in a single AST traversal that produces one diff per file, replacing the previous one-diff-per-error pipeline merged via PhpMerge.

Why

Previously, every RuleErrorBuilder::fixNode() call produced its own unified diff. Patcher::applyDiffs() then merged N diffs per file using PhpMerge::mergeHunks(), looping with a full-file Differ::diffToArray() per hunk. Cost grows quadratically with hunk count.

How

• New BatchReplacingNodeVisitor matches all of a file's pending fixes via origNode attribute in a single NodeTraverser pass.
• Fix callables run eagerly during the rule's pass (Fiber-scope is live, so $scope->getType() works inside callables) and the result node is memoized.
• After all errors for a file are collected, one clone + one batch traversal + one prettyPrintFile + one diff → one FixedErrorDiff per file.
• Nested fixes are handled naturally by PhpParser's traversal: when an outer fix is applied, the traverser descends into the new subtree and applies any inner fixes that the outer's callable preserved. Inner fixes whose original node was discarded are silently skipped.
• Patcher::applyDiffs() is unchanged in shape but only ever sees one diff per file in the new path.

Benchmark

Via new PatcherBenchmarkTest, executed on AMD 9950X3D

hunks	LoC	baseline	refactored	speedup
5	100	2 ms	2 ms	1×
20	500	213 ms	10 ms	21×
50	1000	2.38 s	28 ms	85×
100	1000	4.98 s	37 ms	135×

Real-world test

1977-file project, using two custom fixable rules with 376 violations across 145 files, some of them with 2-4k LoC. Without cache, analysis ran for 53.1s without measurable change from the previous state. Fix ran for 62.3s adding 9.2s on top of analysis.

[phpstan-bot]

You've opened the pull request against the latest branch 2.2.x. PHPStan 2.2 is not going to be released for months. If your code is relevant on 2.1.x and you want it to be released sooner, please rebase your pull request and change its target to 2.1.x.

[mabar]

@phpstan-bot switched to 2.1.x 😘
Remaining CI failures seem unrelated to the changes

[ondrejmirtes]

1. How do you handle traits? Those files might be "fixed" from different usages at different points during the analysis.
2. I'm torn about a problem I didn't realize. Let's say we gave two errors, both fixable, one ignored and one not. Running --fix will fix them both, right? We probably can't solve it becaus we need the one diff per file for performance, and at this point we don't (and should not) be aware what errors are ignored and not.

[ondrejmirtes]

Also, probably, depending on the answer to the previous one, I would not attach the diff to an Error anymore. With one diff per file, it doesn't make sense. So we need a new field for this in the result cache, merging logic in the ResultCacheManager too.

[ondrejmirtes]

At the same time it'd also be useful to track how many errors applying a fix fixes...