chore: remove dead code and fix stale naming (#76, #77)

[LiranCohen]

Summary

Removes 651 lines of dead code and fixes stale naming/comments across the entire codebase, based on findings from the audit (issues #76 and #77).

Dead code removed (#76)

internal/control/

• UpdateFunc / WithUpdateHandler / onUpdate — callback stored but never invoked
• entryMetadata.Tags — populated but never read
• RelayData.RecordID — never populated or read
• NetworkConfig.ListenPort / DefaultRelays — never read
• DNSConfig.Domains — never populated in production

internal/engine/

• Engine.dwnClient, .converter, .sys, .dialer, .autoKeyDelivery — stored but never read after construction
• DWNControl.hostinfo, .netinfo, .endpoints — written, never read
• loadAndPush loginFinished parameter — never used in function body

internal/dwn/

• ProtocolAPI subsystem (~240 lines) — entirely self-referential, never called
• ReadData() / byteReader — never called
• ErrJSONRPC / ErrRecordsWriteWS — never referenced
• 10 unused JSON-RPC error codes
• InterfaceRecordsSubscribe — never dispatched
• dataConsumed field — write-only
• DeriveKeyDeliveryEncryption / DeriveKeyDeliveryWriteEncryption — test-only, no production callers

internal/mesh/

• ServiceIP — exported but never referenced
• RegisterNodeParams.DiscoKey — never written or read (disco key exchanged via endpoint records)
• NodeRegistration.MeshIP — echoes caller's own data, never read
• 4 unused FetchContextKeyParams fields
• "peer remove" in command combiner — no handler exists

Stale naming fixed (#77)

• All "dexnet" references → "meshnet" in control/types.go (9 occurrences)
• "member" → "node" in comments across packages
• Consolidated protocol URI to single protocols.MeshProtocolURI constant — removed control.ProtocolMesh and mesh.protocolMesh, replaced 7 hardcoded literals in cmd/meshd/main.go
• Removed duplicate mesh.PublicEndpoint type — use control.PublicEndpoint everywhere
• Condensed 38-line design journal comment in keydelivery.go to 4 lines

Verification

• go build ./... — zero errors
• go vet ./... — zero warnings
• go test ./... -count=1 -race — all tests pass, no data races

Closes #76, closes #77