net/nat clean-ups

[qmonnet]

Some Claude-assisted clean-ups for the net and nat (first commit) crates, in particular to get rid of the confusing leftovers for "unidirectional" flow keys, now that bidirectional ones are gone.

Based on top of #1548.

[Copilot]

Pull request overview

This PR removes legacy “unidirectional flow key” scaffolding by flattening FlowKey into a single struct and updating NAT/flow-table code to use the simplified API. It also refactors NAT error-to-DoneReason mapping to use From conversions instead of helper functions.

Changes:

• Refactor net::FlowKey from an enum + FlowKeyData/Uni wrapper into a single FlowKey struct with direct accessors and TryFrom<&Packet>.
• Update NAT, flow-filter, and flow-entry code/tests to construct and mutate flow keys via FlowKey::new, setters, and TryFrom<&Packet>.
• Replace NAT translate_error() helpers with From<&…Error> for DoneReason implementations.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
net/src/packet/mod.rs	Updates packet flow-key refresh to use TryFrom<&Packet> (removes Uni wrapper).
net/src/packet/meta.rs	Updates flow-key rewrite when setting src_vpcd to use FlowKey::new and accessors.
net/src/lib.rs	Stops re-exporting removed FlowKeyData.
net/src/flows/flow_key.rs	Core refactor: removes FlowKeyData + enum variant, introduces single FlowKey struct and TryFrom<&Packet>.
net/src/flows/display.rs	Simplifies Display impl by implementing directly on FlowKey.
nat/src/stateless/nf.rs	Refactors error-to-DoneReason mapping via From<&StatelessNatError>.
nat/src/stateful/test.rs	Updates tests to build flow keys via TryFrom<&Packet> (no Uni).
nat/src/stateful/nf.rs	Uses FlowKey::new/accessors and refactors DoneReason mapping via From<&StatefulNatError>.
nat/src/stateful/flows.rs	Updates flow-key field access to new accessors.
nat/src/stateful/allocation.rs	Adds From<&AllocatorError> for DoneReason to centralize mapping.
nat/src/portfw/flow_state.rs	Updates port-forward flow-key construction/mutation to use FlowKey setters/accessors.
nat/src/icmp_handler/icmp_error_msg.rs	Adds From<&IcmpErrorMsgError> for DoneReason to share mapping across NAT modes.
flow-filter/src/tests.rs	Updates tests to use TryFrom<&Packet> flow-key creation.
flow-entry/src/flow_table/table.rs	Updates flow-table tests to construct flow keys via FlowKey::new.
flow-entry/src/flow_table/nf_lookup.rs	Updates flow lookup to use TryFrom<&Packet> and removes unused imports.