[TrapsterDK]

Closes #6179

• build_errors_test runs svelte_tool.mjs build on a fixture with an unclosed tag; asserts non-zero exit and an error naming src/Broken.svelte with line and column.
• render_ssr_test and render_client_test use a component that throws at render; SSR render and client mount both reject with the thrown error.
• Throwing app is its own manual package (app_errors/throwing) because Vite needs index.html at the package root.

Ran the three new tests locally: pass. Nested-Bazel consumer tests not run.

Assisted by Claude.

🤖 Generated with Claude Code

https://claude.ai/code/session_011u9TXHDZrfmzFQ2WT8QAKK