build!: require PHP 8.3

[refsz]

Raises the minimum PHP version to 8.3. Mechanical only — no application code changes, so CI passing is the whole review.

Why now:

• Sputnik forbids itself syntax its users already write. A typed class constant (private const array DEFAULT_SHELL) had to be reverted in EnvironmentDetector because they are 8.3+, while the reference project's own tasks use private const string and private const array freely.
• PHP 8.2 reaches end of security support at the end of 2026, roughly four months out.
• The audience is already past it: Drupal 11 requires 8.3, and the reference project runs 8.4.

What changed:

File	Change
composer.json	"php": "^8.3" and config.platform.php to 8.3.0
composer.lock	refreshed via composer update --lock so composer install does not fail on a stale hash; no dependency versions moved
.github/workflows/ci.yml	matrix drops 8.2; the four pinned jobs move to 8.3
.github/workflows/release.yml	both pinned versions move to 8.3
docs/index.md, docs/installation.md	the two places that state the requirement

Verified locally: composer validate clean, 742 tests passing, PHPStan [OK] No errors.

Follow-up in a separate PR, so this one stays mechanical: the 8.3 features that actually earn their place — the reverted typed constant, and #[Override] on the redacting decorators.

Correction to an earlier claim in this description: I wrote that TemplateRenderer::withFormatter() could become a real clone now. That is wrong, and I checked it rather than repeating it — 8.3 permits reinitialising a readonly property only inside __clone(), which cannot receive the new formatter as an argument. Outside __clone() the assignment still throws Cannot modify readonly property. The current new self(...) stays correct.

CLAUDE.md states PHP 8.2+; that file only exists in the unmerged #29, so the line is corrected there instead of here.

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