Integrate with @storybook/addon-coverage

[BartWaardenburg]

Why

@storybook/addon-coverage already wraps istanbul-lib-instrument (webpack builder) and vite-plugin-istanbul (Vite builder), with auto-detection. Threading an instrumenter option through the addon plumbing would let Storybook users adopt oxc-coverage without code changes once both upstream layers are configured.

Scope

1. Investigate whether the addon can pass through a custom instrumenter to both the webpack and Vite paths, or whether the addon's coverage option needs a new field.
2. Open an upstream feature PR adding coverage.instrumenter (or equivalent) that delegates to the underlying provider.
3. Add a README section under "Coverage stack" with a copy-paste Storybook config example.

Acceptance criteria

• Storybook story coverage produces matching output between the default and the oxc-coverage instrumenter
• Upstream PR opened in storybookjs/addon-coverage
• README example added

Notes

The Vite path already works today via the existing vite-plugin-istanbul 9.0.0 integration; the webpack path is the one that needs new plumbing. Consider scoping v1 of this issue to "document Vite-builder usage" while the webpack path tracks separately.

Effort

Vite-path documentation: ~1 day. Webpack-path upstream PR: 3-5 days (depends on Storybook addon internals).