[Epic] Migrate to PHP 8.5

[igancev]

Goal

Fully migrate the project to PHP 8.5: code dependencies, CI pipelines, and binary builds (Linux, macOS, PHAR).

Context

The project currently requires PHP 8.4. All production and dev dependencies already support PHP 8.5. No deprecated PHP features are used in the codebase — migration is primarily infrastructure/configuration changes.

Branch strategy

All work is done against the feature/php-8.5 branch (not main). Each subtask is a separate PR targeting feature/php-8.5. Once all subtasks are complete, feature/php-8.5 is merged into main as one final PR. This prevents broken releases on main while migration is in progress.

Important for contributors using Claude: when creating a PR for any subtask, set the base branch to feature/php-8.5, not main.

Subtasks (execution order)

1. Build new bin-builder Docker image with PHP 8.5 support #44 — Build new bin-builder Docker image with PHP 8.5 support
2. Update composer.json and CI quality-checks for PHP 8.5 #45 — Update composer.json and CI quality-checks for PHP 8.5 · blocked by Build new bin-builder Docker image with PHP 8.5 support #44
3. Update PHAR release workflow for PHP 8.5 #46 — Update PHAR release workflow for PHP 8.5 · blocked by Update composer.json and CI quality-checks for PHP 8.5 #45
4. Update Linux binary release workflow and Dockerfile for PHP 8.5 #47 — Update Linux binary release workflow and Dockerfile for PHP 8.5 · blocked by Build new bin-builder Docker image with PHP 8.5 support #44, Update composer.json and CI quality-checks for PHP 8.5 #45
5. Update macOS binary release workflow for PHP 8.5 #48 — Update macOS binary release workflow for PHP 8.5 · blocked by Update composer.json and CI quality-checks for PHP 8.5 #45
6. Update project documentation for PHP 8.5 #49 — Update project documentation for PHP 8.5 · blocked by Update composer.json and CI quality-checks for PHP 8.5 #45