Define how custom-network agreement works

[jintekc]

Table 1 allocates 12..14 to "Custom networks" (algorithms.md:33-34) but never says how a custom network is identified or agreed. The nibble carries no chain identity: two resolvers both "configured for" value 12 can bind it to different chains and resolve the same DID against different blockchains, with no error on either side. Custom networks are expected in practice (mutinynet was one before getting value 5, #70), and three slots guarantee collisions between unrelated deployments.

Welcoming feedback and discussion on these proposals.