Use Sendable DNS types.

[jglogan]

• Closes [Request]: Eliminate external DNS library dependencies. #1268.
• The types we were using weren't very usable with Swift 6 structured concurrency.
• Implements just the subset of records that we use.
• Use notImplemented instead of formatError for unknown record types.
• Use pure actor for LocalhostDNSHandler now that we have sendable types.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

[manojmahapatra]

Question: some lookup paths appear to use plain string (no normalization), will this change cause any issue? For example in AttachmentAllocator;

// store
func allocate(hostname: String).. 

// lookup
func lookup(hostname: String) async throws -> UInt32? {
        hostnames[hostname]
}

[jglogan]

@manojmahapatra Thanks for looking at this! I converted it to draft.

This one was done by telling Claude to create the types we needed using the bit-fiddling code that ContainerizationNetlink uses, and I pushed it primarily to back the change up and get something started for the issue.

I still need to review the work myself and sanity check it, but I'll bet I would have missed this!

I'll have a look through the relevant RFCs as I review this myself. This isn't an area I'm super expert in so if you've got experience and suggestions I'd welcome them!

[manojmahapatra]

I'm not an expert in this area either, but I reviewed the PR in detail against current behavior. Overall this is moving in the right direction.

I see two gaps worth addressing here:

• Hostname normalization consistency (e.g. foo vs foo.) across parse/store/lookup paths. (additional focused unit tests will be nice)
• Compression-pointer safety in DNSName decode to avoid malformed packet loops. Something like this maybe;

// Calculate pointer offset from message start
                let pointerLocation = offset
                let pointer = Int(length & 0x3F) << 8 | Int(buffer[offset + 1])
                let pointerTarget = messageStart + pointer
                guard pointerTarget < pointerLocation else {
                    throw DNSBindError.unmarshalFailure(type: "DNSName", field: "compression pointer not prior")
                }
                offset = pointerTarget