feat(crate): scaffold the ADR-013 §4 feature graph and module skeleton

- Declare the decided feature graph in the manifest: sqlite default-on
  (rusqlite 0.40 bundled), postgres (tokio-postgres 0.7 +
  deadpool-postgres 0.14), mem (dependency-free), pg-lo pulling the
  shared pg driver cluster only (no postgres implication, ADR-010 §1),
  ops (alkcall 0.8.1, default-off); local ungated
- Replace the placeholder crate body with the module skeleton: lib.rs
  re-export shape, ungated stubs (hashing, error, backend, facade,
  dispatch, gc, local), engine stubs behind #[cfg(feature)]
  (mem, pg_lo, postgres, sqlite, ops); stubs are doc-comment-only so
  wave-1 tasks own the module bodies

Verification: cargo test green (default); cargo check --all-features
resolves the full graph; cargo check --no-default-features --features
pg-lo resolves with rusqlite provably absent (tree-inverted match) —
the ADR-010 §1 no-implication rule; clippy --all-targets -D warnings
and cargo fmt --check clean.

Task: tasks/scaffold-crate-features.md (wave 1)

src/backend.rs

@@ -0,0 +1,3 @@
+//! The Backend trait contract (ADR-003): opaque byte keys and byte-valued
+//! entries, complete `list()`, and the read-cursor / staged-put I/O seams
+//! (ADR-010) with the crate-internal engine-state companion seam.