feat: add VeriSimDB triple API gateway (REST+gRPC+GraphQL) in V-lang

Three V-lang servers following the v-api-interfaces pattern:
- verisimdb-rest (port 9090): CRUD + batch + query for hexads
- verisimdb-grpc (port 9091): gRPC-Web JSON-over-HTTP for HexadService
- verisimdb-graphql (port 9092): GraphQL with mutations, queries, GraphiQL

Storage is file-based (verisimdb-data/hexads/), configurable via
VERISIMDB_DATA_DIR. This is the gateway that panic-attack's HTTP
storage mode talks to.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: hyperpolymath