Implementation plan: wave-based decomposition; waves 1-2 decomposed (11 tasks)

docs/plans/implementation.md records the wave structure (core ->
substrate fork || pg engine -> sqlite engine -> contract suite ->
release), the decided points (contract-suite layout = option (a),
engine-tests vs equivalence-suite split, no CI, mem-engine/fuzzing
deferrals surfaced), and the review-gate rhythm.

Wave 1 (foundations): workspace scaffold, core errors/validation,
value types, trait surface, contract-suite scaffold (+ADR-022),
review gate.
Wave 2 (substrate fork): fork scaffold/provenance, connection+watcher
port, queue-op re-derivation on contract v1, provenance/floor close,
review gate.

tasks/core-errors-and-validation.md

@@ -0,0 +1,72 @@
+---
+id: core-errors-and-validation
+name: Core error taxonomy and name validation
+status: pending
+depends_on: [scaffold-workspace]
+scope: narrow
+risk: low
+impact: project
+level: implementation
+tags: [wave-1, core]
+---
+
+## Description
+
+Implement the core crate's error taxonomy and the name-validation
+helpers every entry point shares.
+
+**Error taxonomy** (ADR-008 §5, exact): one top-level `Error`,
+`thiserror`-typed, `#[non_exhaustive]` (ADR-017 §3 — mechanically
+enforces catch-all matching). Variants:
+
+- `PayloadTooLarge { limit }` — universal variant; produced pg-side
+  only (the occurrence asymmetry is engine work, not core's — core
+  defines the variant and its doc text)
+- `ReservedName { name }`
+- `InvalidName { name }`
+- `Closed`
+- `Codec`
+- `Database` — the opaque fallback; engine detail rides the source
+  chain (`#[source]`), no engine-specific variants ever minted
+
+**Name validation** (ADR-008 §4): a shared validation helper applied at
+every name-bearing entry point — non-empty (`InvalidName`) and
+reserved-prefix rejection for the `__alkstore_` prefix
+(`ReservedName`). The helper distinguishes the two name classes:
+
+- Shared-namespace kinds (channels, streams, queues, outboxes, locks,
+  schedule names): non-empty + reserved-prefix rejected.
+- Consumer-local identifier classes (stream-consumer names, `owner` on
+  `try_lock`, `worker_id` on claims): non-empty only — a leading
+  `__alkstore_` is legal (ADR-008 §4 third-round annotation).
+
+Also the reserved-string constant: `__alkstore_listener_reconnected__`
+(the one v1-reserved string, ADR-008 §4) — public in core so the pg
+engine and the contract suite reference the same constant.
+
+`Result<T>` alias in the crate's prelude shape. No panics; no
+`unwrap()`/`expect()`.
+
+## Acceptance Criteria
+
+- [ ] `Error` with exactly the six v1 variants, `#[non_exhaustive]`,
+      `thiserror`-derived, `Database` carrying `#[source]`
+- [ ] Validation helper(s) covering both name classes, unit-tested
+      (empty, whitespace-only, reserved-prefix, reserved string
+      itself, legal names)
+- [ ] Reserved-string constant exported
+- [ ] `cargo test -p alkstore`, clippy `-D warnings`, fmt clean
+
+## References
+
+- docs/architecture/decisions/008-contract-v1-pinning.md §4, §5
+- docs/architecture/decisions/017-contract-versioning.md §3
+- docs/architecture/core-contract.md §Errors, §Naming / reserved namespace
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion