Improve asset load error messages

[han4wluc]

Summary

• Add display-safe asset load errors while preserving technical diagnostics in error.details and error.cause
• Surface fetch status, missing URL, empty data, audio decode, font, image, and video root causes
• Document the error contract in the Assets & Loading guide and bump route-graphics to v1.23.3

Validation

• bunx vitest run spec/RouteGraphics.publicApi.spec.js spec/audio/audioAsset.spec.js spec/util/createAssetBufferManager.spec.js
• bun run lint
• bun run build

Notes

• bun run test / pre-push hook currently fails unrelated parser fixture assertions in spec/parser/parseText.test.yaml and spec/parser/parseTextRevealing.test.yaml due local text metric differences such as expected height 20 vs received 19. The focused asset-loading tests pass.