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>

docs/value/object.md

@@ -0,0 +1,84 @@
+# Object
+
+Opaque container for `Value::Object`'s key→value storage, enabling
+alternative backends without call-site changes.
+
+## Design
+
+`Object` wraps the storage for a key→value collection of `Value`s and
+provides a curated set of methods (`get`, `insert`, `remove`, `iter`,
+`iter_sorted`, `cursor`, serde). The backing store is private; callers
+never see or pattern-match on it, so the representation can change
+without rippling through call sites.
+
+Multiple backends can coexist at runtime. Because the backing store is
+private, different `Object` instances in the same process can use
+different implementations — e.g., a lazy DB-backed object for `input`,
+inline small-map objects for SARIF location records, and a regular
+sorted map elsewhere — all interoperating through the same opaque
+type. This is stronger than the typical Cargo-feature-selected backend
+seen in precedent crates.
+
+Iteration is split intentionally. `iter()` makes no ordering promise,
+which lets backends that don't keep entries sorted skip any sort work.
+`iter_sorted()` returns entries in `Value` order and is what
+serialization and `Ord` rely on for deterministic output. Cursor types
+add resumable, incremental traversal for the RVM iteration state
+without leaking iterator internals.
+
+`Ord` and `PartialOrd` are defined against `iter_sorted()` rather than
+derived from the storage. Two `Object`s built on different backends —
+or with different insertion histories — compare equal whenever their
+sorted entries match, so changing the backend never changes observable
+comparison results.
+
+## Precedents
+
+Other crates that hide storage behind a stable API so the implementation
+can change without breaking callers:
+
+- **`serde_json::Map`** — opaque newtype allowing cargo-feature based
+  swap between `BTreeMap` (canonical order) and `IndexMap` (insertion
+  order).
+- **`toml::Table`** — opaque newtype allowing cargo-feature based swap
+  between `BTreeMap` and `IndexMap`.
+- **`simdjson` DOM** — opaque tree that lazily materializes nodes on
+  access instead of parsing the whole document up front.
+
+## Use cases
+
+- **SARIF small-object pressure** — SARIF reports contain millions of
+  small objects (location records, rule references, message arguments),
+  most with 2-5 keys. A small-map-optimized backend (inline storage
+  for ≤N entries, heap above) eliminates per-object BTreeMap allocation
+  for the common case.
+
+- **Kubernetes admission policies** — large, deeply-nested resource
+  objects (Pod specs, CRDs) where policies typically touch a handful
+  of paths. A lazy-materializing backend (`LazyObjectProvider` over
+  the incoming JSON) parses only the accessed subtrees.
+
+- **Azure Policy aliases** — ARM exposes the same logical property
+  under multiple aliases (e.g. paths like
+  `Microsoft.Compute/virtualMachines/storageProfile.osDisk.managedDisk.id`).
+  An alias-aware backend resolves lookups across canonical and alias
+  forms without rewriting every policy.
+
+- **Azure Policy case-insensitive compare** — ARM property names are
+  case-preserving but case-insensitive on lookup (`tags.Environment`
+  and `tags.environment` resolve identically). A case-insensitive
+  backend centralizes this once at the storage layer instead of at
+  every comparison site.
+
+- **External data sources** — `input` or `data` backed by a database
+  query, CBOR slice, REST endpoint, or other streaming source via a
+  `LazyObjectProvider`. Entries materialize on demand; the policy
+  only pays for what it touches.
+
+- **Eval-time temporaries** — objects constructed during evaluation
+  (comprehensions, intermediate rule results) on a bumpalo arena.
+  The whole arena drops at query end with zero per-entry free cost.
+
+- **Host-language interop** — Python dicts or JS objects accessed via
+  FFI callbacks from the embedding application, without copying into
+  Rust on every binding boundary.