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>

src/rvm/vm/errors.rs

@@ -5,6 +5,7 @@ use super::execution_model::SuspendReason;
 use crate::value::Value;
 use alloc::string::String;
 use alloc::vec::Vec;
+use core::time::Duration;
 use thiserror::Error;
 
 /// VM execution errors
@@ -17,7 +18,14 @@ pub enum VmError {
         pc: usize,
     },
 
-    #[error("Execution stopped: exceeded maximum memory limit of {limit} bytes with usage {usage} bytes (pc={pc})")]
+    #[error("Execution exceeded time limit (elapsed={elapsed:?}, limit={limit:?}, pc={pc})")]
+    TimeLimitExceeded {
+        elapsed: Duration,
+        limit: Duration,
+        pc: usize,
+    },
+
+    #[error("Execution exceeded memory limit (usage={usage} bytes, limit={limit} bytes, pc={pc})")]
     MemoryLimitExceeded { usage: u64, limit: u64, pc: usize },
 
     #[error("Literal index {index} out of bounds (pc={pc})")]