Tasks

• Cargo pimdir feature + path deps (io-pimdir/client, io-replica); build.rs backend list; default features.
• Backend::Pimdir + allows_pimdir + Display.
• config: PimdirConfig { root, source } + AccountConfig.pimdir.
• src/pimdir/: client.rs (open store as source + blobs), hash.rs (Neverest-matching content hash), backend.rs (read + write adapter).
• shared/client.rs: import, BackendClient::Pimdir, all dispatch arms, select_storage arm (local-before-network).
• Reads from meta (no body reads); get_message "body not fetched" when level < Full; writes via the mutate seam with a synced-source guard.
• Tests: envelope-from-meta, add link/meta derivation, flag-op algebra, content-hash shape.
• nix develop --command cargo build/test --bins; cargo fmt; clippy clean.
• Fold delta.md into cairn/spec/backends.md; add cairn/log entry; mark change landed.