Replace rkyv/bincode caching with redb indices

Remove three-tier loading (rkyv snapshot, bincode cache, capnp replay)
in favor of direct capnp log replay + redb for indexed access.

- Remove all rkyv derives from types (Node, Relation, enums, etc.)
- Remove Snapshot struct, RKYV_MAGIC, CACHE_MAGIC constants
- Remove load_snapshot_mmap(), save(), save_snapshot()
- Remove MmapView, AnyView from view.rs (keep StoreView trait)
- Simplify Store::load() to just replay capnp logs
- Add db.rs with redb schema: nodes, uuid_to_key, visits, transcript_progress
- Simplify cmd_fsck to just check capnp integrity + graph health

capnp logs remain source of truth; redb indices will be rebuilt on demand.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/hippocampus/store/mod.rs

@@ -1,21 +1,16 @@
-// Append-only Cap'n Proto storage + derived KV cache
+// Append-only Cap'n Proto storage + redb indices
 //
-// Two log files are source of truth:
+// capnp logs are the source of truth:
 //   nodes.capnp     - ContentNode messages
 //   relations.capnp - Relation messages
 //
-// The Store struct is the derived cache: latest version per UUID,
-// rebuilt from logs when stale. Three-tier load strategy:
-//   1. rkyv mmap snapshot (snapshot.rkyv) — ~4ms deserialize
-//   2. bincode cache (state.bin) — ~10ms
-//   3. capnp log replay — ~40ms
-// Staleness: log file sizes embedded in cache headers.
+// redb provides indexed access; Store struct holds in-memory state.
 //
 // Module layout:
 //   types.rs   — Node, Relation, enums, capnp macros, path helpers
 //   parse.rs   — markdown → MemoryUnit parsing
-//   view.rs    — zero-copy read-only access (StoreView, MmapView)
-//   persist.rs — load, save, replay, append, snapshot (all disk IO)
+//   view.rs    — StoreView trait for read-only access
+//   persist.rs — load, replay, append (capnp IO)
 //   ops.rs     — mutations (upsert, delete, decay, cap_degree, etc.)
 //   mod.rs     — re-exports, key resolution, ingestion, rendering
 
@@ -24,6 +19,7 @@ mod parse;
 mod view;
 mod persist;
 mod ops;
+pub mod db;
 
 // Re-export everything callers need
 pub use types::{
@@ -34,7 +30,7 @@ pub use types::{
     new_node, new_relation,
 };
 pub use parse::{MemoryUnit, parse_units};
-pub use view::{StoreView, AnyView};
+pub use view::StoreView;
 pub use persist::fsck;
 pub use ops::current_provenance;