refactor(rules): split angular.md into index + references

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

shared/rules/angular/references/performance.md

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+## Performance
+
+* Use `OnPush`.
+* Use stable tracking in loops.
+* Avoid unnecessary change detection triggers.
+* Avoid repeated expensive template calculations.
+* Avoid unnecessary subscriptions.
+* Avoid unnecessary DOM rendering.
+* Lazy-load large features and heavy dependencies where appropriate.
+* Avoid large initial bundles.
+* Avoid importing full libraries when only a small part is required.
+* Avoid unnecessary deep cloning.
+* Avoid unnecessary object and array recreation in hot paths.
+* Keep state updates targeted.
+* Avoid unnecessary global state updates.
+* Measure bundle size and runtime performance before micro-optimizing.
+* Prefer architectural improvements over template-level micro-optimizations.
+
+## Performance Priority
+
+Prioritize Angular performance work in this order:
+
+* Application architecture
+* Network and API behavior
+* Bundle size and lazy loading
+* State architecture
+* Change detection
+* Rendering and DOM complexity
+* RxJS and subscription behavior
+* Memory allocations
+* CPU micro-optimizations
+
+Never optimize based purely on assumptions.