Resolves the first item ("partial reloads") of #1900.

Summary

Adds server-side support for Inertia.js partial reloads:

• Reads X-Inertia-Partial-Component, X-Inertia-Partial-Data (only), and X-Inertia-Partial-Except headers.
• Accepts function-valued props that are evaluated lazily, so heavy data fetching is skipped for props that the client did not request.

app.get('/', (c) =>
  c.render('Home', {
    fast: 'inline value',
    slow: () => db.expensiveQuery(), // not invoked when excluded by a partial
  })
)

Scope

This PR sticks to the foundation that the rest of the breakdown in #1900 builds on:

• ✅ only / except props filtering
• ✅ Lazy function props (() => T | Promise<T>)
• ✅ Component-mismatch guard: partial headers are ignored when X-Inertia-Partial-Component does not match the rendered component

Deferred to follow-up PRs:

• ② Deferred props (<Deferred>)
• ③ Merge / prepend props
• ④ Infinite scroll
• ⑤ always(fn) marker for one-shot props such as flashed messages

Behavior notes

• A request is treated as a partial reload only when X-Inertia-Partial-Component matches the component being rendered and at least one of X-Inertia-Partial-Data / X-Inertia-Partial-Except is present. Otherwise the request is processed as a normal Inertia visit.
• The renderer stays fully synchronous when no function-valued prop is present, preserving the existing Response (non-Promise<Response>) return type for the common case.
• Function props excluded from the partial render are never invoked.

Backward compatibility

Callers that pass only plain values keep the exact same behavior. The new type ResolvedProps<P> awaits the return type of any function-valued prop — plain props are pass-through.

Tests

9 new tests in packages/inertia/src/index.test.ts covering the partial filter, lazy invocation skip, async resolution, component mismatch, whitespace handling, only+except intersection, and the "partial-only header without component" guard.

The author should do the following, if applicable

• Add tests
• Run tests
• yarn changeset at the top of this repo and push the changeset
• Follow the contribution guide