feat: implement P0/P1 performance and code quality optimizations

P0 Performance Optimizations:
- Add debounced search (300ms) to reduce re-renders during typing
- Implement single-pass filter function for O(n) complexity
- Add TanStack Query cancellation before new requests

P1 Code Quality Optimizations:
- Add runtime type guards for filter validation
- Implement two derived values (filteredFonts + sortedFilteredFonts)
- Remove all 'as any[]' casts from filter bridge
- Add fast-path for default sorting (skip unnecessary operations)

New Utilities:
- debounce utility with 4 tests (all pass)
- filterUtils with 15 tests (all pass)
- typeGuards with 20 tests (all pass)
- Total: 39 new tests

Modified Files:
- unifiedFontStore.svelte.ts: Add debouncing, use filter/sort utilities
- filterBridge.svelte.ts: Type-safe validation with type guards
- unifiedFontStore.test.ts: Fix pre-existing bugs (missing async, duplicate imports)

Code Quality:
- 0 linting warnings/errors (oxlint)
- FSD compliant architecture (entity lib layer)
- Backward compatible store API

src/shared/lib/utils/index.ts

@@ -8,5 +8,6 @@ export {
     type QueryParamValue,
 } from './buildQueryString/buildQueryString';
 export { clampNumber } from './clampNumber/clampNumber';
+export { debounce } from './debounce/debounce';
 export { getDecimalPlaces } from './getDecimalPlaces/getDecimalPlaces';
 export { roundToStepPrecision } from './roundToStepPrecision/roundToStepPrecision';