feat(sql): IS NULL / IS NOT NULL + typed Option<Value> INSERT pipeline (SQLR-7)

[joaoh82]

Summary

• Adds WHERE col IS NULL / IS NOT NULL to the executor (two new eval_expr arms; full-scan fallback since NULLs aren't indexed).
• Refactors the INSERT pipeline from stringly-typed Vec<Vec<String>> (with "Null" as a sentinel) to typed Vec<Vec<Option<Value>>> from the parser through Table::insert_row and validate_unique_constraint.
• Fixes Table::restore_row to accept NULL for INTEGER / REAL / BOOLEAN / VECTOR columns so persisted NULLs reopen cleanly — without this, a saved DB containing any NULL outside TEXT failed to reload.

Bugs fixed

• INSERT INTO t (n) VALUES (NULL) for INTEGER / REAL / BOOLEAN / VECTOR columns errored via "Null".parse::<T>() failing.
• INSERT INTO t (s) VALUES (NULL) for TEXT silently stored the literal string "Null".
• SQL UNIQUE now allows multiple NULLs (standard three-valued logic).
• DEFAULT NULL collapses to "no default" — explicit NULL in INSERT is preserved over the column DEFAULT (matches SQLite).
• Restored SQLR-2's default_does_not_override_explicit_null test that had to be dropped because of this bug.

Test plan

• cargo test --workspace --exclude sqlrite-desktop --exclude sqlrite-python --exclude sqlrite-nodejs — 397 engine tests pass (was 396)
• cargo fmt --all -- --check clean
• cargo clippy --workspace --exclude sqlrite-desktop --exclude sqlrite-python --exclude sqlrite-nodejs --all-targets clean (no new warnings beyond the pre-existing set)
• REPL smoke test: create, insert mixed NULL / non-NULL rows, run IS NULL / IS NOT NULL, close, reopen — values round-trip
• New file-backed null_values_round_trip_through_disk exercises every column type's NULL through save/reload
• 6 INSERT-NULL tests (one per column type: INTEGER / TEXT / REAL / BOOLEAN / VECTOR / JSON)
• 5 IS NULL / IS NOT NULL executor tests (non-indexed, indexed UNIQUE, omitted-column, combined-with-AND)

Out of scope (deferred)

• Removing the legacy Row::Text "Null" sentinel-strip on read (kept as back-compat for pre-fix saved DBs; revisit at next file-format bump).
• Negative numeric literals in INSERT VALUES (pre-existing Expr::UnaryOp(Minus, …) gap).
• IS NULL index optimization (would need a NULL-rowid sidecar; full scan is fine for now).

🤖 Generated with Claude Code