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/dsl/command.rs

@@ -442,6 +442,16 @@ pub enum Command {
         /// Whether a `RETURNING` clause matched (ADR-0033 §5,
         /// sub-phase 3g).
         returning: bool,
+        /// Captured literal RHS of each top-level `SET col = <literal>`
+        /// assignment (ADR-0036 Phase 2). `(col, Some(v))` for a bare
+        /// literal (incl. a signed number); `(col, None)` for an
+        /// expression RHS (arithmetic, function call, scalar subquery,
+        /// column ref — nothing static to validate). The worker validates
+        /// the `Some` values against their column types before the (still
+        /// verbatim) update; the error enricher reads them to name the
+        /// offending value. Execution itself is unchanged — these are
+        /// *not* bound. `WHERE` is deliberately excluded (ADR-0036 §2).
+        set_literals: Vec<(String, Option<Value>)>,
     },
     /// A SQL `DELETE` validated by the walker (ADR-0033 §1/§7,
     /// advanced mode). Grammar-as-text: the worker executes `sql`,