feat: propagate config.platform from core's composer.json

[mstrelan]

This aims to solve https://www.drupal.org/project/drupal/issues/3605599 without waiting for core.

Disclaimer - this is entirely vibe coded.

Below is the description that Claude came up with, which is an accurate summary of what I'm trying to do:

composer-merge-plugin does not merge the config section, only package metadata. This means config.platform.php from core's composer.json is not visible to tools like PHPStan when running via composer.local.json as the root, causing PHPStan to analyse against the running PHP version (e.g. 8.5) instead of the declared target version (e.g. 8.3 on 11.x).

Hoist the core composer.json read out of the installer-paths block and propagate config.platform via Config::merge() alongside it, using the same pattern already used for installer-paths and root version pinning.

I ran some tests on main, as well as 11.x and 11.4.x but it wasn't working, so I had Claude fix it:

The original plugin approach (in-memory Config::merge() only) was insufficient — phpstan reads $COMPOSER env var, finds composer.local.json, and reads config.platform directly from that file in its own process. The fix adds syncPlatformToRootFile() which writes config.platform from composer.json into composer.local.json on disk during every composer invocation. Branch switches work correctly as long as you run ddev composer update after switching, which you need to do anyway.

So now if you switch branch and run ddev composer update it will sync config.platform.php from core in to composer.local.json so phpstan detects it. You can see the output by adding -vvv to the phpstan command:

PHP runtime version: 8.5.5
PHP version for analysis: 8.3 (from config.platform.php in composer.json)

I've also added a bats test to verify.