Module backends 

Pluggable persistent CacheBackend implementations (chartml 5.0 phase 3b).

Each backend lives behind its own cargo feature so non-browser builds stay lean. Today this hosts:

• [codec] — pure-Rust binary codec for the on-disk blob format. Shared across every persistent backend so the framing has one source of truth. Compiled whenever a backend that uses it is on (today: wasm-indexeddb) OR whenever tests are running, so the framing is exercised on native via cargo test --workspace even with no feature flags enabled.
• [indexeddb::IndexedDbBackend] — browser-side persistent cache wired through the [idb] crate. Behind the wasm-indexeddb feature, only compiles on the wasm32-unknown-unknown target.

Future tier-2 backends (file-system, SQLite, Redis, …) will land alongside indexeddb here, each behind their own feature flag, and reuse the same codec framing.