docs: define TLD-free name identifiers and amend the derivation docs

[BigTava]

Summary

• Adds docs/design/name-identifiers.md defining name identifiers, their DotNS namehash form, and the TLD convention.
• The convention makes the full served name the identity: the TLD stays in account derivation, ring contexts, entropy, permissions, and storage, so test and mainnet deployments never share state.
• The reserved built-ins are network-scoped: uid.{tld} and peopl.{tld} follow the new dotns_tld host config (CLI presets, wasm runtimeConfig.dotnsTld, native record field), defaulting to dot so unconfigured hosts keep the mobile-pinned derivations.
• Hosts also declare their network TLD through the optional HostChainSet.tld field in the supported_chains syscall.
• Adds the Previewnet test TLD to DOTNS_TLDS and updates the host CLI SPEC and README.
• Regenerates the TS adapter, codegen golden, and Swift bindings. The mobile app still pins uid.dot in its KeyDerivation package and needs the same rule.