liquidity/script: add Starlark script evaluation package

Add a new package for evaluating Starlark scripts in the autoloop
context. This includes:

- context.go: Types exposed to scripts (ChannelInfo, PeerInfo,
  SwapRestrictions, BudgetInfo, InFlightInfo, AutoloopContext,
  SwapDecision) with Starlark Value interface implementations
- builtins.go: Helper functions loop_out() and loop_in() for
  creating swap decisions in scripts
- starlark.go: Evaluator for executing scripts with context data
- starlark_test.go: Unit tests for script evaluation

Starlark provides Python-like syntax with variables, functions,
list comprehensions, and sorting - making complex swap logic
readable and maintainable compared to expression languages.

Author: sputn1ck

liquidity/script/builtins.go

@@ -0,0 +1,121 @@
+package script
+
+import (
+	"fmt"
+
+	"go.starlark.net/starlark"
+)
+
+// loopOutBuiltin creates a loop out swap decision.
+// Usage: loop_out(amount, channel_ids) or loop_out(amount, channel_ids, priority)
+func loopOutBuiltin(thread *starlark.Thread, fn *starlark.Builtin,
+	args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
+
+	var amount starlark.Int
+	var channelIDs *starlark.List
+	var priority starlark.Int = starlark.MakeInt(1)
+
+	if err := starlark.UnpackArgs(fn.Name(), args, kwargs,
+		"amount", &amount,
+		"channel_ids", &channelIDs,
+		"priority?", &priority,
+	); err != nil {
+		return nil, err
+	}
+
+	// Validate amount.
+	amtVal, ok := amount.Int64()
+	if !ok {
+		return nil, fmt.Errorf("amount overflow")
+	}
+	if amtVal <= 0 {
+		return nil, fmt.Errorf("amount must be positive")
+	}
+
+	// Convert channel IDs.
+	ids := make([]starlark.Value, channelIDs.Len())
+	for i := 0; i < channelIDs.Len(); i++ {
+		ids[i] = channelIDs.Index(i)
+	}
+
+	priorityVal, ok := priority.Int64()
+	if !ok {
+		return nil, fmt.Errorf("priority overflow")
+	}
+
+	// Create the decision dict.
+	dict := starlark.NewDict(4)
+	if err := dict.SetKey(starlark.String("type"),
+		starlark.String(SwapTypeLoopOut)); err != nil {
+		return nil, err
+	}
+	if err := dict.SetKey(starlark.String("amount"), amount); err != nil {
+		return nil, err
+	}
+	if err := dict.SetKey(starlark.String("channel_ids"),
+		starlark.NewList(ids)); err != nil {
+		return nil, err
+	}
+	if err := dict.SetKey(starlark.String("priority"),
+		starlark.MakeInt64(priorityVal)); err != nil {
+		return nil, err
+	}
+
+	return dict, nil
+}
+
+// loopInBuiltin creates a loop in swap decision.
+// Usage: loop_in(amount, peer_pubkey) or loop_in(amount, peer_pubkey, priority)
+func loopInBuiltin(thread *starlark.Thread, fn *starlark.Builtin,
+	args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
+
+	var amount starlark.Int
+	var peerPubkey starlark.String
+	var priority starlark.Int = starlark.MakeInt(1)
+
+	if err := starlark.UnpackArgs(fn.Name(), args, kwargs,
+		"amount", &amount,
+		"peer_pubkey", &peerPubkey,
+		"priority?", &priority,
+	); err != nil {
+		return nil, err
+	}
+
+	// Validate amount.
+	amtVal, ok := amount.Int64()
+	if !ok {
+		return nil, fmt.Errorf("amount overflow")
+	}
+	if amtVal <= 0 {
+		return nil, fmt.Errorf("amount must be positive")
+	}
+
+	// Validate peer pubkey.
+	if len(peerPubkey) == 0 {
+		return nil, fmt.Errorf("peer_pubkey cannot be empty")
+	}
+
+	priorityVal, ok := priority.Int64()
+	if !ok {
+		return nil, fmt.Errorf("priority overflow")
+	}
+
+	// Create the decision dict.
+	dict := starlark.NewDict(4)
+	if err := dict.SetKey(starlark.String("type"),
+		starlark.String(SwapTypeLoopIn)); err != nil {
+		return nil, err
+	}
+	if err := dict.SetKey(starlark.String("amount"), amount); err != nil {
+		return nil, err
+	}
+	if err := dict.SetKey(starlark.String("peer_pubkey"), peerPubkey); err != nil {
+		return nil, err
+	}
+	if err := dict.SetKey(starlark.String("priority"),
+		starlark.MakeInt64(priorityVal)); err != nil {
+		return nil, err
+	}
+
+	return dict, nil
+}