v1.2.0 — TopologyGraph attribute store + Codable snapshot

v1.2.0 — TopologyGraph attribute store + Codable snapshot (closes #168)

MINOR — additive new public API. No breaking changes; safe to take blindly within the v1 line.

TopologyGraph nodes were bare (kind, index) pairs with no payload, and the type had no
serialization (it wraps an opaque C++ handle). This adds a pure Swift-side sidecar so callers can
attach arbitrary typed metadata to any NodeRef and round-trip a whole session. No C++ bridge
change — the store never touches the C++ graph.

extension TopologyGraph {
    public var attributes: NodeAttributeStore
    public func attribute(_ key: String, for: NodeRef) -> AttrValue?
    public func setAttribute(_ key: String, _ value: AttrValue, for: NodeRef)
    public func snapshot() throws -> GraphSnapshot
    public convenience init(snapshot: GraphSnapshot) throws
}

• AttrValue — closed Codable enum: bool / int / double / string / ints / doubles
  (ints for mesh-region index sets, doubles for fitted-surface params).
• NodeAttributeStore — Codable, keyed by NodeRef; encodes as sorted arrays for
  deterministic element order. Pair with GraphSnapshot.canonicalEncoder() (.sortedKeys) for
  byte-stable, diffable output.
• GraphSnapshot — Codable round-trip. The graph structure is re-derived by rebuilding from
  the source shape's BREP (captured at construction); rebuild pins parallel: false. A determinism
  test verifies NodeRef indexing is stable across rebuilds.
• NodeKind / NodeRef gain Codable.

Foundation for the OCCTReconstruct mesh-to-solid
pipeline (per-node fit residual / confidence / provenance + session persistence) and OCCTMCP's
planned reconstruct_* graph read/write tools (OCCTMCP #33).

xcframework unchanged — pure Swift addition. 7 new tests; all 124 TopologyGraph tests pass.