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.

CLAUDE.md

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+# RDBMS Playground — project notes for Claude
+
+## What this project is
+
+A cross-platform TUI application that gives learners a sandbox for
+exploring relational database concepts: tables, columns, primary
+and foreign keys, relationships, indexes, queries, and query
+plans. The audience is students from beginners to those ready for
+raw SQL, and the design accommodates both ends of that spectrum.
+
+The application is a teaching tool, not a database administration
+tool. Decisions about the type system, command surface, and
+backend choices are skewed toward pedagogy over breadth.
+
+## Authoritative decisions
+
+All significant design decisions live in `docs/adr/`. Read
+`docs/adr/README.md` for the index. **Before proposing changes
+that touch a decided area, read the relevant ADR.** Decisions are
+not re-litigated casually — if a decision needs to change, write a
+new ADR that supersedes the old one.
+
+Current decisions at a glance (each backed by an ADR):
+
+- **Stack:** Rust + Ratatui + Crossterm; `sqlparser-rs` for SQL,
+  `rusqlite` for the database (ADR-0001).
+- **Backend:** SQLite with `STRICT` tables and FK enforcement on
+  (ADR-0002).
+- **Input:** simple mode (DSL only) by default; advanced mode
+  (SQL + app-level commands) on toggle; `:` one-shot escape from
+  simple to advanced (ADR-0003). No other sigils.
+- **Project format:** `project.yaml` + `data/<table>.csv` +
+  `history.log`; `playground.db` is a derived artifact (ADR-0004).
+- **Types:** `text`, `int`, `real`, `decimal`, `bool`, `date`,
+  `datetime`, `blob`, `serial`, `shortid`. Compound primary keys
+  supported. No real UUIDs (ADR-0005).
+- **Safety:** auto-snapshot before destructive ops; `:undo`;
+  append-only `history.log` for replay and scripting (ADR-0006).
+- **Sharing:** `export` command produces a zip without the `.db`;
+  no hosted publishing (ADR-0007).
+
+## Repository layout (planned)
+
+The repository is in early planning. Once code lands, layout
+conventions will be added here. For now, `docs/adr/` is the
+canonical source of decided ground.
+
+## Working style for this project
+
+- **Documentation discipline.** Significant decisions get an ADR.
+  In-flight discussion stays in conversation or issues until it
+  settles.
+- **Testing.** Per the user's global standards, tests are
+  established before changes, bugs are reproduced with failing
+  tests before being fixed, and "all green, no skips" is the only
+  acceptable end state. Integration tests exercise full flows
+  (e.g. project save → reload → rebuild produces the same
+  observable database), not just units in isolation.
+- **No silent feature loss.** Anything in an ADR is decided. If
+  implementation reveals that a decision is wrong or impractical,
+  raise it explicitly and update the ADR — do not quietly drift.
+- **Pedagogy wins ties.** When a design choice trades clarity for
+  raw capability, prefer clarity. Real RDBMS power-user features
+  exist; this app is not the place to teach them.
+
+## Things deliberately deferred
+
+These have been discussed but not yet decided. Do not implement
+ahead of an ADR for any of them:
+
+- Tutorial / lesson system architecture (acknowledged as in scope
+  for design; needs its own ADR before code).
+- Query plan rendering specifics (the *what* is decided — annotated
+  tree view of `EXPLAIN QUERY PLAN`; the *how* is not).
+- Friendly error message rewriting layer (decided to exist; design
+  not done).
+- Sample data generation rules, including FK-aware generation for
+  junction tables.
+- Visual ER diagram export.
+- DSL grammar specifics (compound key syntax, m:n relationship
+  command, etc.).