002 budget exhaustion

// Test: Budget exhaustion mid-flow
// When budget runs out, return exhausted branch with what was used

~import std/runtime
~import std/io

const std = @import("std");

~pub tor expensive { data: string }
| processed string

~proc expensive|zig {
    return .{ .processed = data };
}

// Register with high cost
~std/runtime:register(scope: "test") {
    expensive(50)  // costs 50 tokens per call
}

const TEST_SOURCE = "expensive(data: \"a\")\n| processed p1 |> expensive(data: \"b\")\n| processed p2 |> result { p2.result }";

// Run with budget of 75 - second call should exhaust
~std/runtime:run(source: TEST_SOURCE, scope: "test", budget: 75)
| result _ |> std/io:print.ln("FAIL: should have exhausted")
| exhausted e |> std/io:print.ln("PASS: exhausted after {{ e.used:d }} tokens, last event: {{ e.last_event:s }}")
| parse-error _ |> std/io:print.ln("PARSE ERROR")
| validation-error _ |> std/io:print.ln("VALIDATION ERROR")
| shape-error _ |> std/io:print.ln("SHAPE ERROR")
| event-denied _ |> std/io:print.ln("EVENT DENIED")
| dispatch-error _ |> std/io:print.ln("DISPATCH ERROR")
| scope-not-found _ |> std/io:print.ln("SCOPE NOT FOUND")