fix: preserve cached annotations when another term is absent - #955

Summary

• Create annotation dictionaries only for new targets so an absent term cannot evict cached records or merged custom query options.
• Evaluate uncached annotations once while preserving intentional cache resets when the model changes.
• Add regression coverage for first-time and existing targets, repeated absent terms, cached record identity and merged state, model changes, and navigation GET-to-$count custom-option propagation.

Fixes #951.

Validation

• Confirmed all four new regression cases fail before the fix.
• Focused annotation-cache and $count tests: 26 passed.
• Full reader test suite: 1,290 passed.

No endpoint-specific inheritance workarounds, count narrowing changes, optional request-body changes, or pipeline changes.