Release tooling for the PIE + PECL dual channel

[sj-i]

Sets up publishing to both channels so old and new PHP are both covered:
PIE/Packagist for modern setups (the pie tool needs PHP 8.1+, though the
extension builds on 7.0+) and PECL (pecl install rdump) for the older
runtimes PIE itself can't run on.

Added

• .github/workflows/release.yml — on a v* tag (or manual dispatch):
  • verify the version is in sync across php_rdump.h, package.xml, and the
    tag (fails on drift);
  • pecl package-validate;
  • build the extension and run the .phpt suite;
  • pecl package to produce rdump-X.Y.Z.tgz (the exact artifact uploaded to
    pecl.php.net), smoke-install it, and attach it to the GitHub Release.
• CHANGELOG.md — 0.1.0 notes.
• RELEASING.md — the cut-a-release steps plus one-time Packagist
  submission and the PECL proposal/karma onboarding.
• .gitattributes — export-ignore dev-only paths from the dist archive.
• README — clarify PIE (needs 8.1+) vs pecl install (handy on older PHP).

Not automated (needs maintainer account)

• Submitting the repo to Packagist (one-time) and enabling the hook.
• Registering on pecl.php.net and proposing the package on pecl-dev for karma
  (one-time); thereafter uploading the tarball the workflow produces.

These are documented in RELEASING.md.

https://claude.ai/code/session_017skw8BsHkF8wur7Pf4G9W5

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