Resolve #3591829: Coalesce multi-bundle references with differing per-bundle fields

Low-level prop-expression infrastructure for authoring multi-target-bundle content-entity-reference picks whose per-bundle field selections differ. Blocks #3591656, which exposes and renders the feature and owns the constraint/validator changes and the CER integration tests.

What this MR does

• Parser + serializer (FieldObjectPropsExpression): round-trips a bundle-specific branch nested in an object prop (label↝entity␜[branch][branch]). Nested branching stays rejected via the existing parseBranches() depth guard.
• Coalescer: groups standalone references by their referencer field and coalesces each bundle's picks. When every bundle contributes a single referenced expression the result stays a field-level reference or branch (unchanged). When a bundle picks multiple fields — which no single-field branch member can represent — the picks combine into one object on the reference field, grouped by developer-facing key: a key picked in several bundles becomes a bundle-specific branch prop, a key in one bundle a plain reference prop. coalesce(), expand() and idempotency stay lossless.

Everything is dormant on 1.x until #3591656 exposes multi-bundle browsing and removes the save-time constraint.

Testing instructions

• composer run phpunit -- tests/src/Unit/CoalescerTest.php
• composer run phpunit -- tests/src/Unit/PropExpressionTest.php
• composer run phpunit -- tests/src/Kernel/Entity/JavaScriptComponentUpdateFromClientSideTest.php
• composer run phpunit -- tests/src/Kernel/Config/JavaScriptComponentValidationTest.php
• composer run lint (PHPCS + PHPStan)

Notes for reviewers

• The render half is verified end to end on top of #3591656's buildReferencePayload: news_item → {body,label}, blog_post → {label, owner}, bundle-specific props omitted for the non-matching bundle.

AI usage disclosure

Implemented with AI assistance (Claude Code, Claude Opus 4.8), reviewed by the contributor before pushing, per Drupal.org's Policy on the use of AI.