db+walker: 3i DA pass — not_null PK false-positive fix + arity hardening

DA pass on 3i. Fix: build_schema_cache set not_null = c.notnull ||
c.primary_key, which would false-flag an omitted `int` PK as a
not_null_missing WARNING — but an int PK is an INTEGER PRIMARY KEY
rowid alias that auto-fills (and SQLite's PK-NULL quirk means a PK
isn't implicitly NOT NULL anyway). Use c.notnull alone (ADR-0033
§8.3 "declared NOT NULL"): faithful and false-positive-free.

Arity-walk hardening (same class as the ON CONFLICT regression the
existing tests caught mid-3i): RETURNING after VALUES is a depth-0
keyword that ends the tuple list (only the real tuple is flagged),
and a comma nested in a function-call value (depth ≥ 2) does not
inflate the tuple's value count.

Tests (+2). 1598 pass / 0 fail / 1 ignored. Clippy clean.

src/runtime.rs

@@ -1002,12 +1002,15 @@ async fn build_schema_cache(database: &Database) -> crate::completion::SchemaCac
                     c.user_type.map(|ty| TableColumn {
                         name: c.name,
                         user_type: ty,
-                        // A PK column is effectively NOT NULL even when
-                        // PRAGMA's notnull flag isn't set on it
-                        // (ADR-0033 §8.3); the not_null_missing pass
-                        // excludes auto-gen types (serial/shortid)
-                        // separately.
-                        not_null: c.notnull || c.primary_key,
+                        // Only a column the engine reports as NOT NULL
+                        // (ADR-0033 §8.3 "declared NOT NULL"). We do
+                        // NOT treat a PK as implicitly not-null: an
+                        // `int` PK is an `INTEGER PRIMARY KEY` rowid
+                        // alias that auto-fills, so flagging it omitted
+                        // would be a false positive (SQLite reports
+                        // notnull=0 for it anyway). serial/shortid are
+                        // excluded by type in the not_null_missing pass.
+                        not_null: c.notnull,
                         has_default: c.default.is_some(),
                     })
                 })