[perf] Enforce single-pass rule application in tests, make rules idempotent

[TomasVotruba]

Why

FileProcessor::processFile() runs rules in a do { ... } while (true) loop that re-runs the whole file until nothing changes. The loop is needed for legitimate cross-rule composition, but it also hides rules that are not finished in a single pass: a rule that re-applies to its own previous-pass output silently relies on the loop re-running it, instead of resolving the nested or chained case itself.

What

Add a test-only guard in FileProcessor that fails a rule which changes a file again after it already changed it in the previous pass. The loop stays as the runtime safety net; the guard only turns the hidden dependency into a test failure.

• Gated by the RECTOR_ENFORCE_SINGLE_PASS env (set in phpunit.xml), so sibling packages pulling rector-src as dev-main are not affected by it.
• Post rectors are excluded, as import handling legitimately converges over several passes.

Make the rules that depended on the re-run resolve their nested or chained case inside refactor():

• ReplaceConstantBooleanNotRector - fold the whole !! negation chain by parity
• RemoveDeadZeroAndOneOperationRector - reduce the returned operand chain
• PowToExpRector - convert nested pow() calls
• TernaryToNullCoalescingRector - convert a nested coalescable else
• ReplaceBlockToItsStmtsRector - flatten nested blocks
• RemoveAlwaysElseRector - split nested always-else in the returned stmts

ClosureToArrowFunctionRector was already made idempotent on main.

All fixtures keep their current expected output - they now reach it in a single pass. Full suite, ECS and PHPStan are green.

[TomasVotruba]

Heads up: the single red check, packages_tests (rector-downgrade-php, Unit Tests) - DowngradeNullJsonTest, already fails on main (e.g. commit fcdacd4) and is unrelated to this PR. The guard here is gated behind the RECTOR_ENFORCE_SINGLE_PASS env set only in this repo's phpunit.xml, so sibling packages are not affected by it - all other sibling jobs are green.