Issue #3627952: Fix the phpstan jobs, correct the documentation and refresh the codebase metrics for the first 4.x release

Closes #3627952

One commit per step:

1. Next major phpstan. The two @phpstan-ignore lines in DomainSourceToken and the one in DomainConfigHookTest no longer match any error, and both phpstan jobs now fail on them as unmatched: the next major job first, and the current one since its toolchain moved on after the last green 4.x run (every MR against 4.x is red on them, for example !472 (merged)). The token reads the source field through $node->get('field_domain_source') and the test gives its user an explicit password, so neither lane reports an error and no ignore is needed.
2. Documentation. README.md and docs/index.md (English, French, Spanish) state the 4.x requirements (Drupal 11.4 or later, Drupal 12 compatible). The caching section of docs/index.* now says the service provider adds the domain cache context, instead of asking for a services.yml override. The quoted phpstan configuration is the current phpstan.neon (level 3).
3. Metrics. docs/metrics.md and its translations are measured on this branch, rebased onto the #3624218 fix, with cloc 2.08 over git ls-files, leaving out .tugboat/ and the metrics pages themselves. Files outside every module (README, documentation, CI and tooling configuration) are listed as a project root row, so the domain module is not credited with them and the submodule table adds up to the totals. The Methodology section gives the two commands that reproduce the snapshot. These pages must be regenerated again if anything else merges into 4.x before the release is tagged.

AI-Generated: Yes (Claude Code was used to write these changes. I reviewed them before posting.)