fix: isolate config snapshots to eliminate provider map race

[jearthliu]

Fixes the provider-mutates-after-send race with a send-side snapshot contract:

• New Configuration.Snapshot() helper: providers send a deep copy they fully own, so the config through the channel is never shared with provider maps.
• GetConfigurationChan() documents the ownership contract: send a Snapshot, or never mutate after send.
• The watcher still copies before applying (defense-in-depth) and GetConfig() returns a copy.

Consumer-side copying alone cannot stop a provider that mutates a shared map after sending; snapshot-on-send is the actual fix. Closes #1.