feat: master seed key derivation for all roles

[AquiGorka]

Summary

All identity keys now derive from a single master seed. Switching to a different master secret (e.g., a funded mainnet wallet) changes all derived keys consistently.

Changes

• lib/master-seed.ts — derivation module: masterSeedFromSecret(), deriveKeypair(seed, role, index), deriveAllKeys(), role constants, default local-dev master secret
• setup-keys.sh — derives all 7 role keys (admin, pp, opex, pay-admin, pay-service, alice, bob) from one master seed. Override with MASTER_SECRET=S... env var.
• setup-c.sh — reads ADMIN_SK from .local-dev-keys instead of DEPLOYER_SK
• setup-pp.sh — reads PP_SK from .local-dev-keys instead of using hardcoded PP_SECRET
• fund-keys.sh — derives all keys, funds via Friendbot (--friendbot) or shows transfer plan (--transfer)
• testnet/main.ts — accepts MASTER_SECRET env var for deterministic keys; falls back to Keypair.random()
• lifecycle/testnet-verify.ts — same MASTER_SECRET support

Roles

Role	Key	Used for
admin	Council admin / contract deployer	Deploy contracts, create council
pp	PP operator	Register PP, sign bundles
opex	PP treasury / OpEx	Network fees
pay-admin	Pay platform admin	Admin API auth
pay-service	Pay platform service	Inter-service auth
alice	Test user A	Deposit, send
bob	Test user B	Receive, withdraw

Running against mainnet

MASTER_SECRET=S... \
COUNCIL_URL=https://council-api.moonlightprotocol.io \
PROVIDER_URL=https://provider-api.moonlightprotocol.io \
deno task e2e

Test plan

• Master seed module: deterministic derivation verified
• Testnet e2e without MASTER_SECRET: passes (random keys)
• Testnet e2e with MASTER_SECRET: running (deterministic keys)