Maintenance: fix duration bugs, update dependencies, modernise CI

[sifex]

Summary

Full maintenance pass: fixes the two open bug reports, updates all dependencies to current majors, adds static analysis that was referenced but missing, and modernises CI.

Bug fixes — closes #4 and #6

Both issues reported SLA durations missing time on the final day (e.g. 16:58 Fri → 09:01 Mon returned 2 min instead of 3 min 1 sec). Root cause: the per-day iteration in SLA::get_current_duration() and the agenda period generation in Agenda\Weekly::toPeriods() were anchored at the start time-of-day, so when the end time-of-day was earlier than the start time-of-day, the end date was never iterated and its SLA time was dropped.

• src/SLA.php — anchor the daily iteration at startOfDay() and keep the real subject bounds separate for the overlap min/max logic
• src/Agenda/Weekly.php — same day-anchoring fix for agenda period generation
• tests/SLATest.php — regression tests for both reported scenarios

Verified: issue #4 now returns 379 min (was 77), issue #6 returns 181 s (was 120).

Dependencies

• PHP floor raised to ^8.3 (8.1 is EOL, 8.2 EOL Dec 2026) — unlocks the current toolchain
• nesbot/carbon ^2 → ^3
• illuminate/collections → ^11|^12|^13
• Dev: pestphp/pest ^1 → ^4 (PHP 8.4/8.5 compatible — Pest 1 crashes on 8.5), phpstan/phpstan ^1 → ^2, laravel/pint ^1.30, spatie/invade ^2, spatie/pest-plugin-test-time ^2.2; removed direct phpunit/phpunit (bundled by Pest)
• npm: vitepress 1.6.4, tailwindcss/@tailwindcss/vite 4.3.3, vue 3.5.41
• Fixed the broken test-coverage composer script (missing space: pest--coverage-clover)

Static analysis & style

• Added phpstan.neon.dist (CI workflow referenced it but it didn't exist) and fixed all 50 level-8 findings, including a latent bug (Weekly::addTimePeriods over-flattened period pairs) and dead code calling a never-defined method
• Migrated phpunit.xml to the current schema (PHPUnit 12)

CI

• Test matrix: PHP 8.1–8.4 → 8.3, 8.4, 8.5
• First-party actions only: actions/checkout@v7, actions/setup-node@v7, actions/github-script@v9, plus shivammathur/setup-php and GitHub's dependabot/fetch-metadata; removed peaceiris/actions-gh-pages, laravel-pint-action, git-auto-commit-action, changelog-updater-action, ramsey/composer-install (replaced with direct tooling)
• Docs deploy now pushes to gh-pages via plain git

Housekeeping

• docs/guide/getting_started.md — requirements updated to PHP 8.3
• Superseded stale dependabot PRs Bump rollup and vitepress #12 and Bump vite and vitepress #15 (closed)