fix: atomic config swap — no panic, router/middleware always consistent

[notr3kt]

Summary

Fixes #1.

Configuration reloads from concurrent providers could:

1. Panic on first swap (atomic.Value type mismatch: HandlerFunc → *ServeMux)
2. Risk stale / partial handler publish if rebuild and store were not a single consistent snapshot

Fix

• handlerHolder wrapper so atomic.Value always stores one concrete type
• Build complete mux from one config snapshot, then publish under mutex + single atomic store
• Drain burst updates → apply latest only (serialized watcher)
• GetConfig() returns deep copies

Tests

go test ./... -race -count=3

• TestFirstConfigSwapDoesNotPanic
• TestConfigAndHandlerStayConsistent (2000 reqs under dual providers)
• TestGetConfigDeepCopy

Claim

/claim #1

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[notr3kt]

Friendly bump for review when you have a moment.

• go test ./... -race -count=3 green locally
• Fixes atomic.Value panic + consistent router/middleware snapshot publish
• Opire claim already registered for @notr3kt

Happy to iterate on any feedback.