feat(tests): the conformance suite's home — conventions, invariant backlog, harness skeleton (wave 1)

- tests/conformance/README.md: the convention doc — row naming (property-
  stated names, engine dimension is the column), the Contract stamp:
  format (the alkstore ADR-017 §4.2 convention adapted), the one-
  normative-owner-per-property rule, the factory contract (fresh
  isolated instance per open, idempotent teardown, safe over an
  abandoned instance), the SKIP posture (SQL-attached columns skip
  cleanly server-less; acceptance requires green against the server),
  the invariant backlog (the seven store-api.md §Invariants registered
  as the rows-to-grow table with pinning citations), and the mem-first
  column plan (mem is the contract-reference engine, ADR-011 §2)
- harness skeleton, compile-able with zero rows: StoreFactory (fresh
  + isolated + idempotent-teardown contract; the Store associated type
  is wave 2's binding point for the facade type), stamp.rs (the
  marker + convention anchor), backlog.rs (in-code registry with a
  traceability gate asserting INV-1..7 mirror §Invariants in order)
- no engine-specific code; no dependencies added

Verification: cargo test (33 passed across lib + 3 targets),
cargo clippy --all-targets -- -D warnings, cargo fmt --check

tasks/suite-scaffold.md

@@ -1,7 +1,7 @@
 ---
 id: suite-scaffold
 name: Conformance suite — home, convention, and invariant backlog
-status: pending
+status: completed
 depends_on: [scaffold-crate-features]
 scope: narrow
 risk: low
@@ -66,8 +66,37 @@ identically here).
 
 ## Notes
 
-> To be filled by implementation agent
+- The convention doc landed as `tests/conformance/README.md` (normative
+  home) with the in-code anchors: `stamp.rs` (the `Contract stamp:`
+  marker, alkstore ADR-017 §4.2 adapted), `factory.rs` (the
+  `StoreFactory` shape + contract), `backlog.rs` (the backlog as an
+  in-code registry with a traceability gate).
+- The skeleton's wave-2 binding point: `StoreFactory::Store` is an
+  associated type placeholder for the `Store` facade type (which
+  arrives in wave 2 — no public type exists to bind today). Wave 2's
+  mem column binds it; a binding-revealed shape correction is wave
+  2's to make and record in README + factory.rs, per the task text.
+- SKIP posture is pinned as convention text (README §SKIP posture +
+  factory.rs docs) rather than typed machinery: the mechanism lands
+  with the first column that needs it (postgres, wave 4). Mem/sqlite/
+  local never skip.
+- The backlog table cites per-entry: restated invariant, planned row
+  family, pinning texts (spec §/ADR §), first column/wave. INV-4's
+  fleet form is recorded as its own engine-gated family (wave 6).
+- Harness self-checks (4 tests): backlog traceability (order mirrors
+  store-api.md §Invariants 1–7, every field populated), stamp marker
+  identity, and a probe factory proving the trait is implementable
+  with async methods (the alkstore exemplar-against-mock posture,
+  sized to the no-rows state). No engine-specific code; no
+  dependencies added.
 
 ## Summary
 
-> To be filled on completion
+`tests/conformance/` established: README.md (row naming, stamp
+format, one-owner rule, factory isolation + idempotent-teardown
+contract, SKIP posture, the seven-invariant backlog table traceable
+to store-api.md, mem-first column plan), plus the compile-able
+skeleton (factory trait, stamp module, backlog registry) with zero
+rows as expected. Verified: `cargo test` (33 total across lib + 3
+targets), `cargo clippy --all-targets -- -D warnings`, `cargo fmt
+--check` all green.