feat: add PersistentHypergraph disk-backed graph

CLAUDE.md

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+# hypergraph
+
+## After every code change
+
+Run both of these before considering a task done:
+
+```bash
+cargo fmt
+cargo clippy --all-targets --all-features
+cargo nextest run
+```
+
+## Module structure
+
+`mod.rs` files must only contain module declarations (`mod foo;`) and re-exports
+(`pub use`). All type definitions, trait impls, and method implementations
+belong in dedicated subfiles — one logical group per file (e.g. `types.rs`,
+`helpers.rs`, `graph.rs`, `vertices.rs`, `hyperedges.rs`).
+
+## Comments
+
+Do not use ASCII ruler/banner comments as section dividers, e.g.:
+
+```rust
+// ──────────────────────────────────────────────────────────────────────────────
+// Section title
+// ──────────────────────────────────────────────────────────────────────────────
+```
+
+or inline section labels such as:
+
+```rust
+// ── Shortest paths ───────────────────────────────────────────────────────────
+```
+
+Instead, document public functions and methods with idiomatic Rust doc comments
+(`///`). Include `# Errors`, `# Panics`, and `# Returns` sections where
+relevant. Every public method in a `pub` API module must have a doc comment.
+
+## Testing
+
+Every public function and method must have at least one test that exercises it
+directly.