feat: multisig support with partial signing for P-Chain operations

[owenwahlgren]

Summary

• Adds multisig ownership support for P-Chain operations (subnets, validators, delegators)
• Adds partial signing workflow: build tx → sign with key 1 → sign with key 2 → submit
• New pkg/multisig package with JSON-based tx file format for exchanging partially-signed transactions between signers
• New platform tx sign|commit|inspect CLI commands for the signing workflow
• --owners, --threshold, --output-tx flags on subnet commands
• --reward-addresses, --reward-threshold flags on validator/delegator commands

Workflow Example

# 1. Create a 2-of-3 multisig subnet
platform subnet create \
  --owners P-fuji1abc,P-fuji1def,P-fuji1ghi \
  --threshold 2 \
  --output-tx unsigned.json \
  --key-name builder

# 2. First signer signs
platform tx sign --tx-file unsigned.json --key-name signer1

# 3. Second signer signs
platform tx sign --tx-file unsigned.json --key-name signer2

# 4. Submit (any key can submit, tx is already signed)
platform tx commit --tx-file unsigned.json --network fuji

# Inspect at any point
platform tx inspect --tx-file unsigned.json

Architecture

• pkg/multisig/ — tx file format, partial signing helpers, OutputOwners builder
• cmd/tx.go — sign, commit, inspect subcommands
• pkg/pchain/ — config structs now accept optional RewardOwner *secp256k1fx.OutputOwners (backward compatible)
• Leverages avalanchego's native partial signing support (signer leaves empty sigs for missing keys)

Test plan

• go vet ./... passes
• staticcheck ./... passes
• All existing unit tests pass
• New pkg/multisig tests pass (OutputOwners creation, TxFile round-trip, signature status, address parsing)
• E2E test: create multisig subnet on local network
• E2E test: partial sign → sign → commit flow