Initial planning docs: CLAUDE.md and ADRs 0000-0008

Captures up-front design decisions for RDBMS Playground:
stack (Rust + Ratatui + SQLite), input modes, project file
format, type vocabulary, undo snapshots and replay log,
sharing/export, and testing approach. ADR-0000 establishes
the ADR practice itself and mandates index upkeep alongside
any ADR change.

docs/adr/0002-database-engine.md

@@ -0,0 +1,56 @@
+# ADR-0002: Database engine
+
+## Status
+
+Accepted
+
+## Context
+
+The application teaches relational database concepts. Requirements
+on the engine:
+
+- File-based, with a single binary file that can live in the
+  project folder.
+- Real RDBMS feature set: typed columns, primary keys, foreign keys
+  with referential integrity, indexes, query planner output we can
+  expose to learners.
+- Industry adoption — students should leave with a skill that maps
+  to something they will encounter again.
+- Embeddable from Rust without arcane build steps.
+
+Candidates considered:
+
+- **SQLite** — file-based, near-universal adoption, supports FK
+  enforcement, has `EXPLAIN QUERY PLAN`, and since 3.37 supports
+  `STRICT` tables which give proper type enforcement instead of
+  SQLite's traditional loose type affinity.
+- **DuckDB** — file-based, modern, but analytical/columnar by
+  design; would teach analytical instincts that mislead in
+  transactional contexts.
+- **Embedded Postgres (pglite, pg_embed)** — closest to a "real"
+  RDBMS, but adds friction and complicates packaging.
+
+## Decision
+
+Use **SQLite** via the `rusqlite` crate. All tables are created as
+`STRICT` tables. Foreign-key enforcement is enabled per-connection
+(`PRAGMA foreign_keys = ON`).
+
+Simplified user-facing column types (see ADR-0005) are mapped to
+the underlying SQLite STRICT types at parse time.
+
+## Consequences
+
+- Tight, low-friction packaging — `rusqlite` bundles SQLite.
+- `EXPLAIN QUERY PLAN` output is directly usable for the query
+  analysis feature (ADR pending).
+- SQLite's `ALTER TABLE` is limited (e.g. type changes, some
+  drops). Schema evolution will use the rebuild-table technique
+  internally; this is hidden from users in simple mode and exposed
+  honestly in advanced mode.
+- STRICT tables forbid the historical permissive typing students
+  might encounter elsewhere — this is intentional and matches the
+  pedagogical goal.
+- Booleans are stored as `INTEGER` (0/1) per SQLite's STRICT type
+  set; the `bool` user-facing type maps to this and is rendered as
+  `true`/`false` in result views.