[codex] Support error path mapping

[66Ton99]

Supersedes the useful behavior from #113 with a small modern patch against the current codebase.

Plan

• Add a structured JavaScript error object for custom constraints that need Symfony-like atPath() behavior.
• Keep existing constraints that return plain strings backward-compatible.
• Route mapped errors to child form elements and clear routed errors on revalidation.
• Document the API and add focused Jest coverage.

Changes

• Introduces FpJsFormError(message) with optional atPath.
• Converts string validation errors into FpJsFormError during validation.
• Resolves direct and nested child paths from the validated element, falling back to the validated element when the path is unknown.
• Clears the current validation source recursively before each validation pass so routed child errors do not remain after the owner constraint becomes valid.
• Documents error path mapping in src/Resources/doc/3_2.md.

Tests

• npm run test:unit -- FpJsFormValidator.test.js --runInBand
• php /tmp/jsfv-composer.phar validate --strict
• php /tmp/jsfv-composer.phar test
• npm test
• git diff --check