Add Rector 2.6 dev tooling and empty rector.php for PHP 8.2.

[shadyvb]

Fixes XWPENG-47.

Base branch: ticket/XWPENG-46-php-minimum (#1978). Config-only PR — first in the XWPENG-47 Rector modernization stack. Adds Rector 2.6 as a dev dependency and an empty rector.php scaffold so follow-on stacked PRs (#1982–#1986) can register rules and apply transformations incrementally. No plugin source changes in this PR.

Summary

• composer.json — adds rector/rector ^2.6 to require-dev and on-demand scripts:
  • composer lint-rector — rector process --dry-run
  • composer rector — rector process
• composer.lock — locks Rector 2.6.5 (and its PHPStan dependency).
• rector.php — PHP 8.2 baseline config with scan paths for plugin source (stream.php, abilities/, alerts/, classes/, connectors/, exporters/, includes/), standard skip dirs, cache under artifacts/rector, and no rules registered yet (withRules( array() )). Sub-PRs append rules as they land.
• phpcs.xml.dist — excludes artifacts/ so Rector cache is not scanned.

Stacked follow-on PRs (same ticket)

PR	Branch	Applies
#1982	ticket/XWPENG-47-typed-props-classes	Typed properties in classes/
#1983	ticket/XWPENG-47-typed-props-connectors	Connector family typing
#1984	ticket/XWPENG-47-typed-props-alerts	Alert/Exporter typing
#1985	ticket/XWPENG-47-constructor-promotion	Constructor property promotion
#1986	ticket/XWPENG-47-match	switch → match

Test plan

• composer validate passes
• CI Lint/Test and E2E pass on PHP 8.2, 8.3, and 8.4 (no runtime behavior change)
• Reviewer: composer install && composer lint-rector exits 0 with no pending diffs
• Reviewer: confirm artifacts/rector is gitignored / PHPCS-excluded

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

• Fix: N/A — dev tooling only, no user-facing change.
• New: N/A.

Release Checklist

• This pull request is to the master branch.
• Release version follows semantic versioning. Does it include breaking changes?
• Update changelog in readme.txt.
• Bump version in stream.php.
• Bump Stable tag in readme.txt.
• Bump version in classes/class-plugin.php.
• Draft a release on GitHub.

Change [ ] to [x] to mark the items as done.