feat: DuckDB engine support (REF-290)

[therecluse26]

Summary

Adds DuckDB as a fourth first-class engine with 100% feature parity with the existing engines (PostgreSQL, MySQL, SQLite), plus a full offline test suite.

Engine (src/engine/duckdb)

• validate_connection, introspect, execute via the duckdb crate (bundled, no system dependency)
• Read-only defense in depth: file-backed databases open with AccessMode::ReadOnly — DuckDB itself rejects writes even if the parser were bypassed (covered by a session-layer test). :memory: cannot be opened read-only and relies on the capability parser.
• Introspection via duckdb_tables()/views()/columns()/constraints()/indexes(): databases, schemas (DuckDB supports real schemas, default main, --schema filter honored), tables, views, composite PK/FK, unique indexes, table/column comments, and row estimates
• Type mapping: HUGEINT/DECIMAL → string (precision-preserving), DATE/TIME/TIMESTAMP → ISO-8601, BLOB → Base64, LIST/STRUCT/MAP/ENUM/UNION → recursive JSON
• Timeouts via InterruptHandle + timer thread → QUERY_TIMEOUT
• Structured EXPLAIN via EXPLAIN (FORMAT JSON) normalized to ExplainPlanNode

Capability layer

New DuckDB dialect validator: SELECT/CTE/SHOW/DESCRIBE/SUMMARIZE/EXPLAIN/transaction control allowed; PRAGMA allowlist (setter forms rejected); COPY/ATTACH/DETACH/INSTALL/LOAD/EXPORT and all DML/DDL rejected with CAPABILITY_VIOLATION.

Surface wiring

• duckdb: DSN scheme (same path forms as sqlite:, incl. duckdb::memory:)
• CLI (connect wizard + flags, introspect, query), MCP server tools/schemas, and schema-diff dispatch
• duckdb cargo feature included in default all-engines

Output contract

Additive only: "engine": "duckdb" in envelopes. No changes to src/output.rs or existing schemas (schema_drift green).

Test evidence

• cargo test — full suite green, including 24 new engine unit tests, 14 capability tests, 7 DSN tests, and 40 integration tests in tests/duckdb_parity.rs (mirrors the SQLite parity matrix: connect, introspect, allowed/denied queries with state-unchanged verification, max_rows/offset/timeout safety, envelope determinism)
• cargo fmt --check, cargo clippy --all-targets --all-features -- -D warnings — clean
• cargo build --all-targets + cargo build --release — clean
• scripts/test-live.sh — MySQL 8.0/8.4 + PostgreSQL 16 live suites green (connection-resolution paths touched)
• Manual CLI end-to-end: success envelope, write denial, introspect, and read-only open all verified against the compiled binary

🤖 Generated with Claude Code

[therecluse26]

CTO review — approved. Reviewed the full diff (capability layer, DSN parsing, CLI/MCP wiring, 1500-line engine module, parity suite). No blockers or majors — read-only enforcement is properly layered (dialect validator + AccessMode::ReadOnly storage backstop for file DBs), no unsafe code, no runtime panics, max_rows/timeout enforcement matches the SQLite engine pattern, and the 40-test parity suite is deterministic with state-unchanged verification on denied writes. Verified locally: build / fmt / clippy -D warnings / full test suite all green. Pushed one review commit (5136eba) fixing a stale MCP tool description and documenting the InterruptHandle-after-drop safety contract (verified against duckdb crate source). Known parity-consistent minors (interrupt during prepare surfaces as QUERY_FAILED not QUERY_TIMEOUT; no BLOB param binding) match existing SQLite behavior — tracked in REF-290 notes. (Posting as comment: GitHub blocks formal self-approval since the branch shares my git identity.)