chore(node): kill subnet — chain/network vocabulary across node (#116)

* feat(platformvm): CreateSovereignL1Tx — single-tx sovereign L1 launch

Adds a new platformvm tx type that atomically registers a sovereign L1
in one P-chain commit. Replaces what is today the four-step flow:

  CreateNetworkTx + AddChainValidatorTx ×N + CreateChainTx ×K + ConvertNetworkToL1Tx

with one signed tx. After commit, the primary network has a permanent
record of the L1's network ID + initial validator set + chain manifest
+ on-chain validator-manager contract — but it does NOT track-chains
or validate the L1's blocks. The L1 runs its own consensus from
genesis. L2/L3/L4 follow the same pattern recursively.

Type shape:

  type CreateSovereignL1Tx struct {
    BaseTx
    Owner           fx.Owner                       // CreateNetworkTx parity
    Validators      []*ConvertNetworkToL1Validator // genesis validator set
    Chains          []*SovereignL1Chain            // VM ID + genesis blob per chain
    ManagerChainIdx uint32                         // index into Chains[]
    ManagerAddress  types.JSONByteSlice            // validator-manager contract
  }

  type SovereignL1Chain struct {
    BlockchainName string
    VMID           ids.ID
    FxIDs          []ids.ID
    GenesisData    []byte
  }

SyntacticVerify enforces:
  - at least one validator (sorted, unique)
  - at least one chain, ≤ MaxSovereignL1Chains (16)
  - ManagerChainIdx is in range of Chains[]
  - ManagerAddress ≤ MaxChainAddressLength
  - per-chain name + VMID + FxIDs + genesis bounds
  - BaseTx + Owner + each Validator each verify

Wired into:
  - Visitor interface
  - codec (registered as the next tx type after ConvertNetworkToL1Tx)
  - signer + complexity + metrics + executor stubs across all visitor
    implementations

Executor body is stubbed with a TODO. The atomic state transition
(mint new networkID from tx hash, seed validator-manager state,
register each Chain, charge fee) lands in a follow-up PR. This PR is
the type definition + interface plumbing so downstream tools (wallet,
CLI, fee calc, metrics) can target the tx type while the executor is
implemented.

* chore(node): kill subnet — chain/network vocabulary across node

Zero remaining `subnet|Subnet|SUBNET` in node Go source. Per canonical
no-subnet rule.

## Wire types (Go fields only; byte-level wire encoding unchanged)

  message/wire/types.go  TrackedSubnets    → TrackedChains
  message/wire/zap.go    same on Read/Write
  proto/p2p/p2p_zap.go   SubnetUptime alias → ChainUptime
                          (consumes luxfi/proto rename in companion PR)

## Comment scrub

  message/wire/types.go            "(chain, subnet) pair" → "(chain, network) pair"
  network/peer/handshake.go        "primary-network or subnet" → "primary-network or per-chain"
  node/node.go                     "per-subnet"   → "per-chain"
                                   "P→subnet warp" → "P→chain warp"
  genesis/builder/builder.go       "their own subnets" → "their own chains"
  vms/platformvm/client.go         dropped "subnet jargon" reference
  vms/platformvm/service.go        dropped "net / subnet jargon" reference
  vms/platformvm/config/internal.go  "subnet-spawned blockchain" → "per-chain blockchain"

## ICPSubnet (Internet Computer adapter)

  ICPSubnet → ICPNet  (type rename — unrelated to platform subnet,
                        but still a subnet word; killed for consistency)
  map field `subnets` → `nets`

## Examples

  wallet/network/primary/examples/bootstrap-hanzo/main.go:
    --subnet-id  → --network-id (CLI flag)
    existingSubnetID local var → existingNetID
    "subnet ID" log lines → "network ID"
    "SUBNET_ID=" output → "NETWORK_ID="
    "subnet-evm VM ID" → "EVM VM ID"
    All comments rephrased.

  wallet/network/primary/examples/heartbeat-tx/main.go: one comment
    rephrase.

go.work workspace cleanup:
  - Removed ./operator/go entry (placeholder; lux/operator polyglot
    layout pending the cross-repo migration)
  - Removed ./operator entry (mid-migration; nothing to build)

Build clean. Zero `grep -rIn "subnet|Subnet" --include="*.go"` matches.

proto/p2p/p2p_zap.go

@@ -39,7 +39,7 @@ type (
 	Message_BFT                 = p2p.Message_BFT
 	Ping                    = p2p.Ping
 	Pong                    = p2p.Pong
-	SubnetUptime            = p2p.SubnetUptime
+	ChainUptime             = p2p.ChainUptime
 	Handshake               = p2p.Handshake
 	Client                  = p2p.Client
 	BloomFilter             = p2p.BloomFilter