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.

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.