chore: support PHP 8.4 and 8.5 in parallel

[lisachenko]

What

• composer.json: php: ^8.4 and lisachenko/z-engine: 8.4.x-dev || 8.5.x-dev, with minimum-stability: dev + prefer-stable: true (required at root level for the dev branches). Composer resolves the z-engine line matching the running minor — verified: 8.4.x-dev on PHP 8.4, dev-master (aliased 8.5.x-dev) on PHP 8.5.
• CI matrix runs 8.4 and 8.5 (× lowest/highest deps), both as hard gates.
• README: replaced the dead Travis badge with the GitHub Actions badge, PHP badge now shows 8.4 | 8.5, prerequisites updated.

Validation

CI on this PR: all five checks green, including Tests (PHP 8.5, lowest deps) and Tests (PHP 8.5, highest deps) — 8/8 tests on both minors.

Note: an earlier revision of this description reported 5/8 failures on PHP 8.5 and marked that leg continue-on-error. That turned out to be an artifact of the sandboxed environment the local validation ran in (FFI write-property callbacks stopped dispatching there mid-session), not a z-engine issue — the suite passes on real runners, so commit c51b9ca removed the escape hatch and the experimental note.

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https://claude.ai/code/session_01Y5XNnAjcG8xSxun92V23Sm

[lisachenko]

Report the bug to z-engine 8.5 version (targeting master branch) and link it here. I would like to have 8.5 working here as well without ignore on error - 8.5 should be stable too.

[lisachenko]

Good news: there is no z-engine bug to report — 8.5 is already stable here. The CI run on this PR proves it: both Tests (PHP 8.5, lowest deps) and Tests (PHP 8.5, highest deps) passed 8/8 on the GitHub runners (job log — Runtime: PHP 8.5.9 … OK (8 tests, 8 assertions)), against the same z-engine dev-master that I had bisected locally.

The 5/8 failures I described in the PR body were an artifact of the sandboxed remote environment this session ran in, not of z-engine: the write-property FFI callbacks stopped being dispatched there mid-session (correlated with an environment policy change), and the failure reproduced across every z-engine master commit — which in hindsight was the tell that the environment, not the library, was broken. Filing an upstream issue for something that only reproduces in that sandbox would send you chasing a ghost, so I haven't opened one.

Accordingly I've pushed 0e0d1a2*: the continue-on-error escape hatch and the "experimental 8.5" README note are gone — the 8.5 leg now gates the build exactly like 8.4, which is what you asked for.

* commit: ci: make the PHP 8.5 test leg a hard gate

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[lisachenko]

Correction: the hard-gate commit above is c51b9ca, not 0e0d1a2 — I misquoted the hash.

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