Add UCI controls for PhoenixNet NNUE

[KnightStackSys]

Adds UCI options for loading, enabling, disabling, and reloading PhoenixNet networks without restarting IronPhoenix.

New UCI options

• EvalFile (string): load a .nnue network from a path
• UseNNUE (check): enable NNUE or unload it and fall back to HCE
• ReloadNNUE (button): reload the current EvalFile

Behavior

• changing EvalFile loads the network immediately and enables NNUE
• UseNNUE false unloads the network and returns evaluation to HCE
• UseNNUE true loads the remembered EvalFile if necessary
• evaluator changes clear the transposition table so HCE/NNUE scores are not mixed
• startup environment/default loading remains unchanged; the UCI shell reflects the currently loaded network/path

Example:

uci
setoption name EvalFile value C:\\nets\\ironphoenix.nnue
setoption name UseNNUE value true
isready

This PR is isolated from IIR/NMP search work.