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.

tests/sql_dml_e2e.rs

@@ -114,8 +114,14 @@ fn run_update(
     input: &str,
 ) -> Result<UpdateResult, DbError> {
     match parse_command(input).unwrap_or_else(|e| panic!("parse {input:?}: {e:?}")) {
-        Command::SqlUpdate { sql, target_table, returning } => rt.block_on(
-            db.run_sql_update(sql, Some(input.to_string()), target_table, returning),
+        Command::SqlUpdate { sql, target_table, returning, set_literals } => rt.block_on(
+            db.run_sql_update_with_literals(
+                sql,
+                Some(input.to_string()),
+                target_table,
+                returning,
+                set_literals,
+            ),
         ),
         other => panic!("expected Command::SqlUpdate from {input:?}, got {other:?}"),
     }