docs(agglayer): add Section 6 - Faucet Registry to SPEC

[mmagician]

Adds a new section to the AggLayer bridging spec covering faucet registration for both bridge-in (non-native ERC20 tokens) and bridge-out (Miden-native tokens), including metadata hash documentation and gaps.

Summary

• Section 6.1: How non-native ERC20 faucets are created and registered on Miden via CONFIG_AGG_BRIDGE notes
• Section 6.2: How Miden-native tokens would be registered on EVM chains (bridge-out path), including open design questions
• Section 6.3: Metadata hash format, dual purpose (leaf verification + wrapped token deployment), and correctness argument

Highlights implementation gaps:

• On-chain metadata hash verification during registration (needs token name storage + abi.encode in MASM)
• Token name storage in faucet (separate PR in progress)
• Miden-native faucet registration flow for outbound bridging (needs design)