feat(rvm): new instructions and loop semantics for Azure Policy support (#659)

The Rego VM was designed around Rego's semantics, but Azure Policy needs
a few things Rego doesn't: host-supplied context alongside input/data,
undefined-to-null coercion for missing fields, skip-undefined collection
behavior for wildcard aliases, and non-vacuous iteration over non-array
values.

This commit adds five new instructions to bridge those gaps:

  LoadContext / LoadMetadata — give programs access to host-supplied
  evaluation context and cached program metadata at runtime.

  ArrayPushDefined — like ArrayPush but silently drops undefined values,
  so wildcard alias collection (field[*].property) excludes absent
  nested properties instead of leaking undefined entries into the array.

  ReturnUndefinedIfNotTrue — early return with Undefined when a guard
  condition isn't satisfied, without tripping a VM assertion failure.
  This models "condition doesn't match" cleanly.

  CoalesceUndefinedToNull — turns Undefined into Null in-place so that
  downstream builtins see null rather than short-circuiting on undefined.

The loop engine also gains an Azure Policy mode: when the source language
is "azure_policy", an Every loop over a non-array value (scalars, null,
objects) iterates once over a virtual Null element instead of being
vacuously true.  This matches how field[*] behaves on non-array fields
in Azure Policy — the condition body runs once against Null, which
typically evaluates to false.

On the plumbing side: the VM gets a context field with set_context(),
metadata is cached as a Value on program load, and map_limit_error is
inlined into memory_check since it had only one call site.

Four new YAML test suites (~880 lines) cover the new instructions and
context/metadata loading, along with instruction parser, display, and
assembly listing support for everything added here.

src/rvm/instructions/mod.rs

@@ -54,6 +54,16 @@ pub enum Instruction {
         dest: u8,
     },
 
+    /// Load host-supplied context value into register
+    LoadContext {
+        dest: u8,
+    },
+
+    /// Load program metadata value into register
+    LoadMetadata {
+        dest: u8,
+    },
+
     /// Move value from one register to another
     Move {
         dest: u8,
@@ -206,6 +216,16 @@ pub enum Instruction {
         value: u8,
     },
 
+    /// Push element to array, but skip if the value is undefined.
+    ///
+    /// Used by Azure Policy's `field('alias[*].property')` wildcard collection
+    /// so that absent nested properties are excluded from the collected array
+    /// rather than producing undefined entries.
+    ArrayPushDefined {
+        arr: u8,
+        value: u8,
+    },
+
     /// Create array from registers - returns undefined if any element is undefined
     ArrayCreate {
         /// Index into program's instruction_data.array_create_params table
@@ -254,6 +274,25 @@ pub enum Instruction {
         mode: GuardMode,
     },
 
+    /// Return undefined immediately when the condition register is not exactly
+    /// `Bool(true)`.  Any other value — including `false`, `Undefined`, `Null`,
+    /// numbers, strings, etc. — causes an immediate return of `Undefined`.
+    ///
+    /// This is used by Azure Policy compilation to model "condition does not match"
+    /// without treating it as a VM assertion failure.
+    ReturnUndefinedIfNotTrue {
+        condition: u8,
+    },
+
+    /// Replace Undefined with Null in a register.
+    ///
+    /// Azure Policy treats missing fields as null rather than undefined.
+    /// This instruction prevents the RVM's undefined-propagation from
+    /// short-circuiting subsequent builtin calls.
+    CoalesceUndefinedToNull {
+        register: u8,
+    },
+
     /// Start a loop over a collection with specified semantics - uses parameter table
     LoopStart {
         /// Index into program's instruction_data.loop_params table