feat: support net.IP/net.IPMask/net.IPNet/[]net.IP across flags, env, and config

[leodido]

Summary

• implement net-family exact-type support
• add built-in define/decode hook coverage for:
  • net.IP
  • net.IPMask
  • net.IPNet
  • []net.IP

Changes

• register net-family define hooks in internal/hooks/define.go
• register net-family decode hooks in internal/hooks/decode.go
  • reuse mapstructure built-ins for net.IP and net.IPNet
  • add custom decode hooks for net.IPMask and []net.IP
• add net pflag value implementations in values/net.go
• update validation to treat built-in-hook struct types as leaf/scalar fields (needed for net.IPNet)
• update env key generation so hooked struct-backed fields can bind env vars (needed for net.IPNet)
• add focused integration tests in internal/hooks/hooks_test.go for flag/env/config and invalid-input cases
• add code comments clarifying CSV parsing semantics and the validation leaf-type rule

Testing

• go test -count=1 ./...