## Linked issue
Closes#4840
## Summary / motivation
Adds Vitest V8 coverage for TypeScript/Svelte tests via
`@vitest/coverage-v8`.
Introduces `just test-ts --coverage` and `just test-ts --coverage
--html`,
and wires TypeScript into the `just test --coverage` umbrella —
completing
coverage support across all three stacks (Python, Rust, TypeScript).
The threshold is set to 5% — intentionally low because the Vitest test
count is small relative to the TypeScript/Svelte source surface. It is
meant to be raised as more tests are added.
## How to test
```sh
# Existing behavior unchanged
just test-ts
# Terminal summary + enforces 5% line coverage threshold
just test-ts --coverage
# Terminal summary + HTML report under out/coverage/typescript/
just test-ts --coverage --html
# Full umbrella — all three stacks
just test --coverage
just test --coverage --html
```
### Checklist
- [x] I ran `./ninja check` or an equivalent relevant check locally.
### Details
- `@vitest/coverage-v8` pinned at `3.2.4` in `package.json`.
- Reports are written to `out/coverage/typescript/` via
`--coverage.reportsDirectory=../out/coverage/typescript` (relative to
the `ts/` working directory where vitest runs).
- V8 provider is preferred over Istanbul: faster and requires no Babel
transform for TypeScript projects.
- Coverage measures only code reachable through Vitest's module graph —
Svelte component rendering is not covered.
- The `yarn` justfile variable is added for platform-aware yarn
invocation (Windows vs Unix).
## Before / after behavior
Before: no `just test-ts`, no TypeScript coverage.
After: `just test-ts` runs Vitest via ninja; `just test-ts --coverage`
runs with V8 instrumentation.
`just test --coverage` now spans all three stacks.
---------
Co-authored-by: Abdo <abdo@abdnh.net>
