feat(value): introduce Set storage abstraction (#740)

Add an opaque `Set` newtype paralleling `Object`, living under
`src/value/set/` with the same module structure (`mod.rs` /
`iter.rs` / `serde.rs`). `Set` wraps `BTreeSet<Value>` today but
exposes only a curated surface: `contains`, `insert`, `remove`,
`iter`, `iter_sorted`, `cursor` (resumable), `is_subset`,
`intersection`, `difference`, serde, and a hand-written `Ord`.
The cursor types are re-exported behind the `rvm` feature so the
follow-up `IterationState::Set` swap can land additively.

To free the `Set` name for the new public type, the crate-internal
`BTreeSet as Set` / `HashSet as Set` aliases in `lib.rs` are
renamed to `MapSet`. All in-tree consumers of the old alias are
updated in lockstep.

`Value::Set` is unchanged in this commit (still wraps
`Rc<BTreeSet<Value>>`); the payload swap and call-site migration
ship in the next PR.

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

docs/value/set.md

@@ -0,0 +1,79 @@
+# Set
+
+Opaque container for `Value::Set`'s element storage, enabling alternative
+backends without call-site changes. Pairs with [`Object`](object.md) under
+a shared design philosophy.
+
+## Design
+
+`Set` wraps a `BTreeSet<Value>` today but exposes only a curated method
+surface (`contains`, `insert`, `remove`, `iter`, `iter_sorted`, `cursor`,
+`is_subset`, `intersection`, `union`, `difference`, serde). The inner set is
+private — callers cannot pattern-match it or hand out references to the
+backing store, so the backend can change without churn at the ~400 call
+sites that name `Set`.
+
+Two iteration methods reflect a real distinction: `iter()` makes no
+ordering promise (lets future hash/lazy backends skip sorting work);
+`iter_sorted()` guarantees deterministic order (used by serialization and
+`Ord`). Cursor types support incremental traversal needed by the RVM
+iteration state without exposing iterator internals.
+
+`Ord` is hand-written against `iter_sorted` rather than derived, so two
+backends that store elements differently still compare equal when their
+sorted contents match.
+
+## Scenarios enabled
+
+- **Hash-backed storage** — `FxHashSet`-backed inner turns O(log n)
+  membership checks into O(1); swap in for policies where elements aren't
+  compared ordinally.
+- **Lazy/streaming** — wrap a `LazySetProvider` (DB query, CBOR slice,
+  REST endpoint) and materialize elements on demand.
+- **Arena allocation** — bumpalo-backed inner for eval-time temporaries;
+  drop the whole arena at query end with zero per-element free cost.
+- **FFI-backed** — host-language collections (Python set, JS Set) without
+  copying into Rust.
+- **Bloom-filter pre-check** — front a large backing set with a Bloom
+  filter for fast negative-membership tests on read-mostly allowlists.
+
+## Known use cases
+
+- **Azure Policy allowed-values lists** — large allowlists (allowed
+  regions, allowed SKUs, allowed image publishers) compared against
+  single resource values. Hash-backed Set turns O(log n) membership
+  checks into O(1).
+- **SARIF rule deduplication** — collapsing duplicate rule references
+  across thousands of result records. Set-of-objects with structural
+  hashing avoids the BTreeSet sort cost on every insert.
+- **RBAC role membership** — checking whether a principal belongs to any
+  of dozens of role groups. Hash-backed Set scales to thousands of
+  members with constant-time membership.
+- **Azure Policy denied-resource-type sets** — exclusion lists used by
+  deny-effect policies; same hash-backed pattern as allowed-values.
+
+## Precedents
+
+- **`indexmap::IndexSet`** — opaque newtype that pairs hash lookup with
+  insertion-order iteration; precedent for "Set with alternative
+  ordering semantics behind a stable surface."
+- **`hashbrown::HashSet`** — backs Rust's `std::collections::HashSet`
+  and demonstrates a fully swappable backend behind a stable API.
+- **`roaring::RoaringBitmap`** — bitmap-backed integer set. Not
+  applicable to `Value` keys directly, but a precedent for the broader
+  idea of "Set with alternative storage representations chosen by
+  workload shape."
+- **`serde_json`** — note that `serde_json` has no Set equivalent: its
+  Value enum collapses sets into arrays. Regorus's first-class Set with
+  storage abstraction is therefore unusually well-positioned among JSON
+  value libraries.
+
+## Notes
+
+Cursor types are `pub` (referenced by public `IterationState`) but not
+re-exported at the crate root. The crate-internal `Set`/`Map`/`MapEntry`
+aliases for `BTreeSet`/`BTreeMap` in `lib.rs` were renamed to
+`MapSet`/`Map`/`MapEntry` when this type landed, to free the `Set` name
+for the new public type. Future Array and String abstractions follow the
+same shape — see `docs/value/array.md` and `docs/value/string.md` when
+they land.