chore(deps): require stable z-engine releases instead of dev branches

[lisachenko]

What

z-engine now ships stable tags for both supported PHP minors — 8.4.2 on the 8.4 line and 8.5.0 on the 8.5 line — so this package no longer has to consume development branches.

• "lisachenko/z-engine": "8.4.x-dev || 8.5.x-dev" → "~8.4.2 || ~8.5.0"
• root "minimum-stability": "dev" / "prefer-stable": true dropped

Why the tilde and not a caret

One z-engine release line per supported PHP minor: ~8.4.2 admits patch releases inside the 8.4 line but never the 8.5 line, and ~8.5.0 does the same for 8.5. A caret would span both lines and any future one, so a z-engine built for PHP 8.6 could be installed here before this package claims to support 8.6.

Resolution stays automatic: each z-engine tag declares its own platform requirement (~8.4.0 for 8.4.x, ~8.5.0 for 8.5.x), so only one line can satisfy a given runtime, and Core::init() still enforces the exact match at boot.

Dropping the stability pair matters more here than elsewhere — this package is tagged, so a consumer installing lisachenko/immutable-object no longer inherits minimum-stability: dev through its dependency tree.

Side effect worth noting: the CI lowest leg now resolves real tags (8.4.2 on PHP 8.4, 8.5.0 on PHP 8.5) instead of a branch tip, so the floor it tests is a version that actually exists and stays fixed between runs.

Testing

PHP	z-engine resolved	Result
8.4.19	8.4.2	8/8 .phpt pass
8.5.9	8.5.0	8/8 .phpt pass

Note: PHPUnit could not be installed in the environment this was prepared in (egress is restricted to the lisachenko/* repositories, so Packagist dist archives for dev dependencies are unreachable). The .phpt files were executed by a runner that reproduces what PhptTestCase does — child process per file, display_errors=1, exact --EXPECT-- comparison, unanchored --EXPECTREGEX-- match. CI runs the real vendor/bin/phpunit on both minors.

Unrelated finding while verifying (no action in this PR)

On a build with opcache.enable_cli=1, the whole immutability guarantee silently disappears on PHP 8.5: ImmutableHandler::install() hooks ImmutableInterface via setInterfaceGetsImplementedHandler(), and with opcache warm the implementor is linked without that handler running, so setWritePropertyHandler() is never installed and $object->publicProperty = 42 just succeeds — no exception, no warning.

Reproduced against both 8.5.x-dev and 8.5.0 (identical commits), so it predates this change and is not caused by it. It disappears entirely with -d opcache.enable_cli=0, which is how CI runs and how the tests above were run. The sibling packages document the same property from the other direction ("op_arrays compiled before the hook existed are served as-is until invalidated"), but here the failure mode is silent rather than merely inert, which seems worth an issue of its own — happy to open one with the minimal reproducer if useful.

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