Matrix 4.0 modernization release

[Thavarshan]

Summary

Modernizes Matrix for the 4.0 major release around predictable cancellation, fail-fast concurrency, stronger typing, lifecycle observability, bounded metrics, and reliable release tooling.

Included

• PHP 8.3+ with ReactPHP Event Loop 1.6 and Promise 3.3
• Native promise cancellation and removal of CancellablePromise
• Iterable/key-preserving collection APIs and helper/facade parity
• Lifecycle operation IDs/types, timeout telemetry, bounded metrics
• PHPUnit 12.5, PHPStan 2.2, Pint, Composer check workflow
• PHP 8.3/8.4/8.5 Ubuntu matrix, Windows smoke, lowest dependencies, audit/archive checks
• README and UPGRADING.md migration documentation

Validation

• composer check
• composer validate --strict
• composer audit
• Composer archive creation

This is intentionally a draft pending CI and dependent-package testing before beta.1.