feat!: add cooperative execution-time limits across engine, VM, and binding (#539)

- Introduce ExecutionTimer/ExecutionTimerConfig to allow limiting evaluating time.
- To amortize time checking costs, checking interval can be configured via the notion of work units
- A global fallback time limit can be set to universally limit all evaluation in addition to engine level limit setting.
- Implement limnits in interpreter and RVM. In RVM, also handle suspend/resume so that time during pause is not counted.
- Add engine-level APIs to set/clear per-engine timer configuration and apply global fallback defaults.
- Surface execution-time limits through FFI and C# bindings
- Add C# tests and example usage to validate engine overrides, global fallback behavior, and compiled policy enforcement.
- Expand docs for execution-time limit
- Add interpreter YAML cases and VM unit tests for time-limit behavior and deterministic time sources.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

bindings/csharp/Regorus/ExecutionTimerConfig.cs

@@ -0,0 +1,62 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System;
+
+namespace Regorus
+{
+    /// <summary>
+    /// Managed representation of the execution timer configuration used by the engine.
+    /// </summary>
+    public readonly struct ExecutionTimerConfig
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ExecutionTimerConfig"/> struct.
+        /// </summary>
+        /// <param name="limit">Maximum wall-clock duration allowed for evaluation. Must be non-negative.</param>
+        /// <param name="checkInterval">Number of work units between timer checks. Must be non-zero.</param>
+        /// <exception cref="ArgumentOutOfRangeException">Thrown when <paramref name="limit"/> is negative or <paramref name="checkInterval"/> is zero.</exception>
+        public ExecutionTimerConfig(TimeSpan limit, uint checkInterval)
+        {
+            if (limit < TimeSpan.Zero)
+            {
+                throw new ArgumentOutOfRangeException(nameof(limit), "Execution timer limit must be non-negative.");
+            }
+
+            if (checkInterval == 0)
+            {
+                throw new ArgumentOutOfRangeException(nameof(checkInterval), "Execution timer check interval must be non-zero.");
+            }
+
+            Limit = limit;
+            CheckInterval = checkInterval;
+        }
+
+        /// <summary>
+        /// Maximum wall-clock duration allowed for an evaluation.
+        /// </summary>
+        public TimeSpan Limit { get; }
+
+        /// <summary>
+        /// Number of work units between timer checks.
+        /// </summary>
+        public uint CheckInterval { get; }
+
+        internal Regorus.Internal.RegorusExecutionTimerConfig ToNative()
+        {
+            if (Limit < TimeSpan.Zero)
+            {
+                throw new InvalidOperationException("Execution timer limit must be non-negative.");
+            }
+
+            ulong ticks = checked((ulong)Limit.Ticks);
+            ulong limitNanoseconds = checked(ticks * 100UL);
+
+            return new Regorus.Internal.RegorusExecutionTimerConfig
+            {
+                limit_ns = limitNanoseconds,
+                check_interval = CheckInterval,
+            };
+        }
+    }
+}