feat: ADR-0036 Phase 2 — validate advanced-mode UPDATE SET literals + retain the value

Mirror Phase 1's capture-at-parse technique on the UPDATE SET assignment
list. build_sql_update calls the new capture_set_literals (data.rs), which
walks the matched tokens (no reparse, no grammar change) and classifies
each top-level `SET col = <rhs>` as a literal (Some, incl. signed numbers)
or an expression (None), using paren depth so a comma inside a function
call or a `where` inside a scalar subquery is not mistaken for a boundary,
and the trailing top-level WHERE is excluded.

Command::SqlUpdate gains set_literals; do_sql_update validates the literals
against their column types via the shared impl_value_for before the still
verbatim update; user_value_for_column reads them so a constraint error
names the offending value. WHERE stays unvalidated; execution and command
identity are unchanged.

Also corrects the stale data.rs header comment (DSL typed slots are wired,
not "deferred") and flips ADR-0036 + README to Phases 1–2 implemented.

Tests: 1934 passing (+4), 0 failed, 0 skipped, 1 ignored; clippy clean.

src/runtime.rs

@@ -1583,6 +1583,14 @@ fn user_value_for_column(command: &Command, column: &str) -> Option<crate::dsl::
             let idx = listed_columns.iter().position(|c| c == column)?;
             literal_rows[0].get(idx).cloned().flatten()
         }
+        // ADR-0036 Phase 2: a SQL UPDATE retains its captured `SET`
+        // literals, so a constraint error can name the real value.
+        // Assignments are explicitly named, so (unlike SqlInsert) there is
+        // no positional/multi-row ambiguity — mirror the DSL `Update` case.
+        Command::SqlUpdate { set_literals, .. } => set_literals
+            .iter()
+            .find(|(c, _)| c == column)
+            .and_then(|(_, v)| v.clone()),
         _ => None,
     }
 }
@@ -2270,8 +2278,9 @@ async fn execute_command_typed(
             sql,
             target_table,
             returning,
+            set_literals,
         } => database
-            .run_sql_update(sql, src, target_table, returning)
+            .run_sql_update_with_literals(sql, src, target_table, returning, set_literals)
             .await
             .map(CommandOutcome::Update),
         // A SQL `DELETE` (advanced mode; ADR-0033 §1/§7). Grammar-