refactor: wire protocol cleanup - remove sender fields from messages

This commit applies all wire protocol cleanup changes from PR #2169
on top of the rebased PR #2167 base:

- Remove sender field from GetMsg, PutMsg, SubscribeMsg, UpdateMsg, ConnectMsg
- Use upstream_addr for routing responses instead of embedded sender fields
- Delete transient_manager.rs (no longer needed)
- Update freenet-macros code generation for new message structure

The routing logic now derives the response target from the connection's
observed address (upstream_addr) rather than trusting sender fields in
messages. This is more reliable for NAT traversal scenarios.

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Co-Authored-By: Claude <[email protected]>

Author: sanity