(17a) net: LLDP TLV framing and the mandatory TLVs

[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 ← this PR
• (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

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

This PR: TLV framing and the mandatory TLVs

The first of three PRs adding net::lldp, a type-safe, byte-lossless parser and builder for LLDP data units.

• TLV framing (tlv): the 7-bit type and 9-bit length header, and RawTlv.
• Octet strings (octets): Octets<MIN, MAX> carries each field's length bounds in its type.
• Network addresses (address), shared by the chassis ID, port ID and management address.
• The mandatory records: chassis ID, port ID, time to live, and the end marker.
• Display for the identifiers, which show lldp neighbors uses later.

The design rules, which hold for the whole module:

• Invariants live in the types. A subtype is implied by the enum variant rather than stored beside its value.
• Parsing is byte-lossless. Every value deparses to the octets it came from, which property tests check at every level.
• Reserved values are rejected, but where the standard is soft the parser stays permissive.
• Serde goes through the checked constructors, so deserialising can't bypass them.

It lives in net rather than a crate of its own because it needs crate-private pieces (LengthError has no public constructor) and reuses Mac, the IP types and the bolero scaffolding.

Doc comments that mention Lldpdu are plain code spans here. Lldpdu arrives in the third PR, which turns them into links.

Testing

Property tests for every type: round trip, parse ∘ deparse = identity, and rejection of reserved values.

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[codecov]

Codecov Report

❌ Patch coverage is 83.92008% with 169 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
net/src/lldp/tlv.rs	74.83%	72 Missing and 3 partials ⚠️
net/src/lldp/address.rs	79.43%	29 Missing ⚠️
net/src/lldp/chassis_id.rs	90.59%	19 Missing ⚠️
net/src/lldp/octets.rs	85.03%	19 Missing ⚠️
net/src/lldp/port_id.rs	91.27%	15 Missing ⚠️
net/src/lldp/ttl.rs	88.34%	12 Missing ⚠️

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