fix: update … --all-rows falls back to the DSL instead of misparsing (ADR-0033 Am4)

Advanced-mode `update T set x = 42 --all-rows` parsed the `--all-rows`
DSL flag as the arithmetic `42 - -all - rows` over phantom columns
`all`/`rows` (Amendment 3's counter-example), masked only by the engine's
`--` comment leniency. The playground supports no `--` line comment, so
this was a misparse (ADR-0027: flag input known to fail at runtime).

Fix: walk_punct refuses a `-` that begins an adjacent `--`. Only the SQL
expression uses Node::Punct('-'), so this is scoped to it. The SET
expression then stops, the SQL UPDATE shape fails, and dispatch falls
back to the DSL Update { AllRows } — symmetry with delete … --all-rows.

Behaviour: `42 --all-rows` → DSL Update{AllRows}; spaced `42 - -3` stays
SqlUpdate (= 45, preserved); adjacent `42--3` → parse error (contrived;
no `--` comment support).

Tests: inverted parse test (+ arithmetic-preserved + adjacent-error
assertions); new full-pipeline update_all_rows_flag_in_advanced_updates_every_row.
Suite 1963/0/1; clippy clean.

src/dsl/walker/driver.rs

@@ -356,6 +356,21 @@ fn walk_punct(
 ) -> NodeWalkResult {
     let bytes = source.as_bytes();
     if position < bytes.len() && bytes[position] == ch as u8 {
+        // ADR-0033 Amendment 4: a `-` does not match when it begins an
+        // adjacent `--`. The playground supports no `--` line comment, and
+        // `--` is the DSL flag marker (e.g. `--all-rows`); treating `--`
+        // as two minus operators silently mis-parsed `set x = 42
+        // --all-rows` as arithmetic over phantom columns `all`/`rows`.
+        // Refusing here makes the SQL expression stop, so the SQL shape
+        // fails and dispatch falls back to the DSL flag. Spaced `- -3` is
+        // unaffected (the dashes are not adjacent). `Node::Punct('-')` is
+        // used only by the SQL expression grammar, so this is scoped to it.
+        if ch == '-' && bytes.get(position + 1) == Some(&b'-') {
+            return NodeWalkResult::NoMatch {
+                position,
+                expected: vec![Expectation::Punct(ch)],
+            };
+        }
         path.push(MatchedItem {
             kind: MatchedKind::Punct(ch),
             text: ch.to_string(),