perf: remove unnecessary async from createBoundaryConventionElement

[benfavre]

Summary

• Remove async keyword from createBoundaryConventionElement in create-component-tree.tsx — the function body is entirely synchronous (zero await expressions)
• Remove await from all 3 call sites (not-found, forbidden, unauthorized boundary elements)
• Eliminates ~30 unnecessary Promise allocations + microtask ticks per request (3 calls per segment, ~10 segments per typical render tree)

Details

createBoundaryConventionElement was declared async but its body only performs synchronous operations: createElement calls, property access, string concatenation, and conditional expressions. The async keyword caused every call to wrap the return value in a Promise and schedule a microtask tick (~2-4μs each), adding unnecessary overhead to the hot dynamic render path.

Test plan

• Existing e2e tests for app-dir pass (not-found, forbidden, unauthorized boundaries still render correctly)
• No behavioral change — the function was already returning synchronously; the Promise wrapper was pure overhead

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[benfavre]

Performance Impact

Problem: createBoundaryConventionElement is declared async but has zero await in its body. It's a fully synchronous function returning [element, path]. Called 3× per segment with await:

await createBoundaryConventionElement({...}) // notFound
await createBoundaryConventionElement({...}) // forbidden  
await createBoundaryConventionElement({...}) // unauthorized

Each await on a synchronous async function:

1. Creates a Promise object wrapping the return value
2. Schedules a microtask to resolve it
3. Suspends the caller until the next microtask tick
4. Resumes the caller with the unwrapped value

Cost: ~2-4μs per unnecessary await. With 3 calls × 10 segments = 30 unnecessary Promise + microtask cycles = 60-120μs per request.

Fix: Remove async from the function. Remove await from all call sites.

Regression Safety

Zero risk — the function body is entirely synchronous. Removing async changes the return type from Promise<[ReactNode, string?]> to [ReactNode, string?]. All call sites are updated to not await.

Test Verification

• 195 tests across 13 suites, all passing