Introduce Object storage abstraction (#735)

Add an opaque Object type for the key→value storage backing
Value::Object. It exposes a small set of methods (get, insert, remove,
iter, iter_sorted, cursor, serde) and keeps the backing store private,
so future representations -- inline small-map, hash-backed, lazy,
arena, FFI-callback -- can plug in without touching the call sites
that name this type.

Nothing in the engine uses Object yet. Value::Object still wraps
Rc<BTreeMap<Value, Value>>; the payload swap and call-site migration
come in the next PR. Object stands on its own unit tests in the
meantime.

docs/value/object.md walks through the design, the precedents it
follows (serde_json::Map, toml::Table, simdjson DOM), and the
concrete workloads the abstraction is meant to unlock.

A matching Set abstraction follows in a separate PR.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/scheduler.rs

@@ -569,7 +569,7 @@ impl Analyzer {
                         }
                         Ok(false)
                     }
-                    Array { .. } | Object { .. } => Ok(true),
+                    Expr::Array { .. } | Expr::Object { .. } => Ok(true),
                     _ => Ok(false),
                 })?;
                 Ok(true)
@@ -666,7 +666,7 @@ impl Analyzer {
                 Ok(false)
             }
             // TODO: key vs value for object binding
-            Array { .. } | Object { .. } => Ok(true),
+            Expr::Array { .. } | Expr::Object { .. } => Ok(true),
             _ => Ok(false),
         })?;
         Ok(vars)
@@ -853,7 +853,7 @@ impl Analyzer {
                 Ok(false)
             }
             // TODO: Object key/value
-            Array { .. } | Object { .. } => Ok(true),
+            Expr::Array { .. } | Expr::Object { .. } => Ok(true),
             _ => {
                 non_vars.push(e.clone());
                 Ok(false)