(17g) lldp: fast start, shutdown ordering, and a required datapath namespace

[daniel-noland]

Native LLDP for the gateway. The dataplane owns its NICs, so nothing else can speak LLDP for them, and lldpd/lldpad are hostile to automation. LLDP itself is small, so the dataplane transmits and receives it.

This used to be one PR. It is now a stack of seven, each building on the one before:

• (17a) (17a) net: LLDP TLV framing and the mandatory TLVs #1906 net: TLV framing and the mandatory TLVs
• (17b) (17b) net: model the optional LLDP TLVs #1920 net: the optional TLVs
• (17c) (17c) net: assemble LLDP data units and decode them in Headers::parse #1921 net: the LLDPDU and Headers::parse
• (17d) (17d) lldp: the transmitter and the neighbour table #1922 lldp: the transmitter and the neighbour table
• (17e) (17e) dataplane: advertise the gateway with LLDP on every port #1923 dataplane: advertise on every port
• (17f) (17f) dataplane: hear LLDP neighbours and show them #1924 dataplane: hear neighbours, show lldp neighbors, all-multicast
• (17g) (17g) lldp: fast start, shutdown ordering, and a required datapath namespace #1925 fast start, shutdown ordering, required datapath namespace ← this PR

Stacked on (16) #1905, capture. (18) #1907, rate limiting, and (19) #1919, traffic protection, build on top.

This PR: fast start, shutdown ordering, and a required datapath namespace

1. Fast start on a new neighbour (802.1AB newNeighbor). When the table learns a neighbour, that port's transmitter fast-starts, so the newcomer learns about us as quickly as we learned about it.
2. The shutdown frame now goes out before the workers stop. drain_in_order used to cancel the workers first, so a shutdown frame sent by a mgmt task never left. lifecycle gains a protocols subsystem, drained before the workers. On stop, the transmitter waits up to 500 ms for its injection queue to drain.
3. A separate datapath namespace is required. Init always isolates both drivers' interfaces, so the runtime's no-bridge branch was dead. A shared namespace is now a startup failure, and the bridge, and with it LLDP, is always built.

Testing

• Transmitter, in virtual time: a new neighbour restarts fast start, doesn't lengthen one already running, and waits for the link while silent. Stopping waits for the shutdown frame to leave, but a frame that never leaves doesn't hold up stopping.
• lifecycle: protocols see the workers still running while they shut down.

Not covered

• No VTEP/loopback fallback for the management address. Deliberately skipped.
• 802.1AB is cited by section number, not with duvet. The standard is free to read under IEEE GET but not redistributable, and our duvet setup vendors the spec text into the repo.

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[codecov]

Codecov Report

❌ Patch coverage is 70.78189% with 71 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
dataplane/src/runtime.rs	0.00%	26 Missing ⚠️
lldp/src/transmitter.rs	75.86%	11 Missing and 3 partials ⚠️
dataplane/src/drivers/cpbridge.rs	0.00%	13 Missing ⚠️
dataplane/src/lldp_agent.rs	75.51%	12 Missing ⚠️
lldp/src/sink.rs	0.00%	6 Missing ⚠️

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