3.0: Auto-detect changes and produce tests on demand

[crazywhalecc]

Overall we need a maintainer command like dev:gen-test to produce matrix for specific PR, rather than changing test-extensions.php manually.

Building on this, we might then be able to introduce more intelligent tests, such as triggering a full integrity test when modifications to PackageInstaller are made.

[crazywhalecc]

• Add nts/zts field definitions and joint validation to ConfigValidator
• Fill in nts/zts defaults (true) in PackageConfig at read time
• Implement dev:gen-test-matrix subcommand (GenTestMatrixCommand.php). Output JSON fields: php, os, extensions, with_libs, zts, with_suggests, ignore_cache
• Write .github/workflows/tests-v3.yml
  - lint: cs-fixer → lint-config → phpstan → phpunit
  - gen-matrix: call dev:gen-test-matrix and expose outputs
  - build: download (with downloads/ cache + --ignore-cache) → build → upload log/ and *.build.json
  - comment (L3 only): post test summary comment to PR
• Add existing config/pkg/ext/*.yml entries that are known NTS-only or ZTS-only with the new fields

Proposal

Instead of a fixed test matrix, all matrix generation logic lives in a new bin/spc dev:gen-test-matrix subcommand: it inspects the PR's changed files and labels. Tests are tiered by change scope — small package changes run a lightweight matrix (2 PHP versions, 1 OS); core framework changes or the run-full-tests label trigger a full matrix across all PHP versions and OSes.