fix(renderers): warn on missing renderer override

[ai-hustle-bro]

Fixes #82.

Summary

• warn when rendererProp names a renderer id that is not present in the registry
• include the requested renderer id and the available registry ids in the warning
• preserve the existing fallback behavior by continuing through canRender/priority/default renderer selection
• split renderer resolution into a testable helper and cover registered vs missing override paths

Validation

• corepack yarn vitest run src/hooks/useRendererRegistry.test.ts
• ./node_modules/.bin/tsc.cmd -b
• corepack yarn eslint src/hooks/useRendererRegistry.tsx src/hooks/useRendererRegistry.test.ts

Note: corepack yarn compile passes the TypeScript build step on this Windows checkout, then fails at the final Unix-only chmod +x ... command.