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 }

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.

src/dispatch.rs

@@ -0,0 +1,2 @@
+//! Size-threshold dispatch between the kv and large tiers (ADR-003):
+//! routing as a pure function of content length.

src/error.rs

@@ -0,0 +1,3 @@
+//! The error family (ADR-012 §6.5): one enum with members at two
+//! altitudes — the trait's effective members plus the facade's
+//! GC-refusal and key-validation variants.

src/facade.rs

@@ -0,0 +1,2 @@
+//! The store facade value types: `EntryMeta`, `Pin`/`Batch`,
+//! `SweepReport`, `GcAbortCause`, and `PinState`.

src/gc.rs

@@ -0,0 +1,2 @@
+//! GC and namespace mechanism (ADR-005/012): registered liveness,
+//! pin-before-publish, delete windows, and sweep outcomes as data.

src/hashing.rs

@@ -0,0 +1,3 @@
+//! Hashing & keys (ADR-002): `Digest`/`Key` types, the canonical
+//! git-blob-sha-256 derivation, tolerated `git-blob-sha-1`, and the
+//! algorithm-tagged byte encoding.

src/lib.rs

@@ -1 +1,22 @@
-pub fn placeholder() {}
+pub mod backend;
+pub mod dispatch;
+pub mod error;
+pub mod facade;
+pub mod gc;
+pub mod hashing;
+pub mod local;
+
+#[cfg(feature = "mem")]
+pub mod mem;
+
+#[cfg(feature = "pg-lo")]
+pub mod pg_lo;
+
+#[cfg(feature = "ops")]
+pub mod ops;
+
+#[cfg(feature = "postgres")]
+pub mod postgres;
+
+#[cfg(feature = "sqlite")]
+pub mod sqlite;

src/local.rs

@@ -0,0 +1,3 @@
+//! The `local` large-tier engine (ADR-003/008): flat
+//! `{hex-prefix}/{hex-prefix}/{hash}` sharding, stage-then-commit-rename
+//! staged puts, and pread-based read cursors.

src/mem.rs

@@ -0,0 +1,2 @@
+//! The `mem` kv-tier engine (ADR-011 §2): a `BTreeMap`-shaped ephemeral
+//! implementation — the contract-reference engine.

src/ops.rs

@@ -0,0 +1,2 @@
+//! The ops surface (ADR-001): alkcall-backed network operations behind
+//! the `ops` feature gate.

src/pg_lo.rs

@@ -0,0 +1,2 @@
+//! The `pg-lo` large-tier engine (ADR-009): postgres Large Objects
+//! storage with descriptorless `lo_get` read windows over companion rows.

src/postgres.rs

@@ -0,0 +1,2 @@
+//! The `postgres` kv-tier engine (ADR-007): content-addressed rows over
+//! a pooled postgres connection, fleet-valid.

src/sqlite.rs

@@ -0,0 +1,2 @@
+//! The `sqlite` kv-tier engine (ADR-003): WAL + `synchronous=NORMAL`
+//! durability, prepared statements, bounded reads.

tasks/scaffold-crate-features.md

@@ -1,7 +1,7 @@
 ---
 id: scaffold-crate-features
 name: Crate scaffold — ADR-013 §4 feature graph and module skeleton
-status: pending
+status: completed
 depends_on: []
 scope: narrow
 risk: low
@@ -65,8 +65,43 @@ outside tests, no comments in code (doc comments on public API fine).
 
 ## Notes
 
-> To be filled by implementation agent
+- Feature graph declared exactly per ADR-013 §4's table:
+  `default = ["sqlite"]`; `sqlite = ["dep:rusqlite"]`;
+  `postgres = ["dep:tokio-postgres", "dep:deadpool-postgres"]`;
+  `pg-lo` pulls the same shared pg driver cluster by `dep:` reference
+  only (no feature implications — the `postgres` feature name appears
+  nowhere in its declaration); `mem = []`; `ops = ["dep:alkcall"]`.
+  `local` ungated, no feature, no dependency.
+- Driver versions verified at implementation time (crates.io + the
+  alkstore engine manifests as the family baseline): `rusqlite = "0.40"`
+  (`bundled`; resolves 0.40.2, matching alkstore-sqlite's baseline),
+  `tokio-postgres = "0.7"` (0.7.18), `deadpool-postgres = "0.14"`
+  (0.14.2), `alkcall = "0.8.1"` (the exact pin the task specified —
+  present on the public registry).
+- Module skeleton: `lib.rs` re-export shape with one module per file —
+  ungated stubs `hashing`, `error`, `backend`, `facade`, `dispatch`,
+  `gc`, `local`; gated stubs `mem`, `pg_lo`, `postgres`, `sqlite`,
+  `ops` behind `#[cfg(feature)]`. All stubs are doc-comment-only;
+  `pub fn placeholder()` removed. Stub content stays empty so the
+  wave-1 implementation tasks (core-hashing-keys, core-error-model,
+  core-trait-and-types) own their module bodies.
+- `Cargo.lock` is untracked in this repo (workspace-wide exclude,
+  consistent with all prior commits) — left untracked.
+- No CI wiring added (manual CI per docs/plans/implementation.md).
 
 ## Summary
 
-> To be filled on completion
+Crate scaffold complete per ADR-013 §4: the full decided feature
+graph (`sqlite` default-on with rusqlite bundled; `postgres`, `mem`,
+`pg-lo` default-off; `ops` with alkcall 0.8.1; `local` ungated) is
+declared in the manifest with inert optional driver dependencies, and
+the module skeleton (lib.rs re-export shape + 12 doc-comment-only
+module stubs, engine modules behind `#[cfg(feature)]`) replaces the
+placeholder body. All acceptance criteria verified: `cargo test` green
+with defaults (rusqlite only non-base dep in the default tree),
+`cargo check --all-features` resolves the whole graph including
+alkcall, `cargo check --no-default-features --features pg-lo` resolves
+with rusqlite provably absent (`cargo tree -i rusqlite` matches
+nothing) proving pg-lo implies no other engine feature, and
+`cargo clippy --all-targets -- -D warnings` + `cargo fmt --check`
+are clean.