refactor(sketch-db): merge simple_map_store into sketch_db module tree - #33

The sketch DB has two structurally-layered pieces that have been
living as sibling modules:
  - `src/stores/sketch_db/`     (logical layer: schemas, timelines, backfill)
  - `src/stores/simple_map_store/` (physical layer: per-key storage backend)

This sibling layout hid the dependency direction: `sketch_db`
depends on the `Store` trait implemented by `SimpleMapStore`, not
vice versa. The physical store is one concrete implementation of
the sketch DB's storage abstraction, not a peer concept.

Move `simple_map_store/` INTO `sketch_db/` as a submodule so the
module tree reflects the layering: everything sketch-DB-related,
including its physical backend, lives under one umbrella.

## What changed

- `git mv asap-query-engine/src/stores/simple_map_store → asap-query-engine/src/stores/sketch_db/simple_map_store`
- `stores::simple_map_store::*` → `stores::sketch_db::simple_map_store::*` across 20 files (16 backend + 1 bench + 3 internal self-references inside the moved directory).
- `stores/mod.rs` drops the `pub mod simple_map_store` declaration but keeps `pub use ... SimpleMapStore` re-export so `crate::stores::SimpleMapStore` still works for all callers.
- `sketch_db/mod.rs` adds `pub mod simple_map_store` + `pub use simple_map_store::SimpleMapStore`.

## What didn't change

- `crate::stores::SimpleMapStore` public path — callers that import via the top-level `stores::` continue to work unchanged.
- `Store` trait still lives at `stores::traits` — it's the neutral interface both the logical layer and the physical impl share.
- No code logic changed. No tests added or removed.

## Test plan

- [x] Pure rename, zero logic delta.
- [x] 647 lib tests pass (same count as PR #32).
- [x] `cargo clippy --workspace --all-targets --tests -- -D warnings` clean.
- [x] `cargo fmt -- --check` clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>