feat(narrative): on-demand loading architecture and async canon entity search

[lark-x]

Summary

• Transformed initial loading of Narrative Workbench to purely load \�ootstrap\ (outline, diagnostics, and revision) instead of eager loading all candidates, choices, and full canon.
• Re-architected \useNarrativeCanonLookupStore\ to provide on-demand caching and async search via \GET /api/v2/worlds/:storyWorldId/narrative/search.
• Upgraded \CanonEntityPicker.vue\ with 250ms debounced async search and loading spinner.
• Added comprehensive unit tests in \useNarrativeCanonLookupStore.test.ts.