fix: resolve MISSING_EXPORT warnings for optional hooks exports

[FrankFMY]

Summary

Fixes #13508

When a hooks.js or hooks.client.js file exists but doesn't export all possible hooks (e.g., only reroute without transport), Vite/Rollup emits MISSING_EXPORT warnings during build:

.svelte-kit/generated/client-optimized/app.js: "transport" is not exported by "src/hooks.ts"

Root cause

The generated app.js uses namespace imports (import * as universal_hooks from '...') and then accesses individual properties like universal_hooks.reroute or universal_hooks.transport. When a property isn't exported, Rollup detects this statically and warns.

A previous workaround (#13687) suppressed these warnings via onwarn, but the root cause remained.

Fix

Spread namespace imports into plain objects:

// Before
import * as universal_hooks from '...';
// universal_hooks.transport → MISSING_EXPORT warning

// After  
import * as _universal_hooks from '...';
const universal_hooks = { ..._universal_hooks };
// universal_hooks.transport → no warning (plain object, not namespace)

Rollup only checks property access on namespace imports (import * as ns). By spreading into a plain object, subsequent property access is no longer analyzed for missing exports.

This also removes the onwarn suppression workaround and the now-unused isRolldown variable.

Note: the server-side already handles this correctly via await import() destructuring (supported in the node18.13 target). The client-side targets chrome87/safari14 which don't support top-level await, making the spread approach the appropriate solution.

Test plan

• Unit tests pass (pnpm -F @sveltejs/kit test:unit)
• pnpm run lint — clean
• pnpm run check — 0 errors
• Integration tests with full hooks (basics — reroute + transport): 17 passed
• Integration tests with partial hooks (hash-based-routing — reroute only, no transport): 10 passed
• Integration tests with partial hooks (options-2 — transport only, no reroute): 1 passed
• Verified generated code with dedent for all edge cases (no hooks, client-only, universal-only, both)

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🦋 Changeset detected

Latest commit: c7a964d

The changes in this PR will be included in the next version bump.

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Name	Type
@sveltejs/kit	Patch

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