fanout processor: defer pdata clone to dispatch time

[lalitb]

Description:

Currently the fanout processor clones pdata for all destinations upfront during register_inflight. This wastes allocations when:

• Sequential mode: only one destination is active at a time
• Fallbacks: may never be triggered if origin succeeds

Optimization: Defer clone+subscribe to dispatch_ready, cloning only when a destination is actually dispatched

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Originally posted by @lalitb in #1878 (comment)