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)

Cargo.toml

@@ -17,7 +17,20 @@ exclude = []
 [lib]
 name = "alkblobs"
 
+[features]
+default = ["sqlite"]
+sqlite = ["dep:rusqlite"]
+postgres = ["dep:tokio-postgres", "dep:deadpool-postgres"]
+mem = []
+pg-lo = ["dep:tokio-postgres", "dep:deadpool-postgres"]
+ops = ["dep:alkcall"]
+
 [dependencies]
 tokio = { version = "1", default-features = false, features = ["rt", "sync", "io-util", "macros"] }
 thiserror = "2"
-parking_lot = "0.12"
+parking_lot = "0.12"
+
+alkcall = { version = "0.8.1", optional = true }
+deadpool-postgres = { version = "0.14", optional = true }
+rusqlite = { version = "0.40", features = ["bundled"], optional = true }
+tokio-postgres = { version = "0.7", optional = true }