feat: H1a CROSS JOIN ON teaching message; advanced-SQL gaps re-verified (ADR-0042)

Empirically re-checking ADR §3's advanced-SQL "gaps" reversed two of
three — the code survey that produced the list was wrong:
- INSERT…SELECT column-count: already handled (verdict=Error, "the
  column list names N column(s) but M value(s) are given";
  insert_select_arity_mismatch_fires).
- RETURNING scope: already handled (completion offers the table's
  columns; `returning <unknown>` → unknown_column diagnostic).

The one genuine residual is fixed: `select … cross join b on …`
rejected the ON with a bare "expected end of input". Add
parse.cross_join_no_on — "a CROSS JOIN has no ON clause — it pairs
every row; for a join condition use `JOIN … ON`, or filter with
`WHERE`" — rendered when the failing token is `on` and the most
recent consumed join is a CROSS join (a precise signature: every
other join requires `on`, so `on` is expected there, not a failure).
Render-only in format_walker_error; two misfire guards locked (plain
join still asks for ON; a stray `on` with no join does not fire).

ADR-0042 §3 corrected + Implementation-outcome records the advanced-SQL
re-check and the user-confirmed low-priority residual (submit-time
expression first-set at non-projection positions, where typing-time
completion already offers the right candidates).

Full suite green (lib 1578 / it 388 / typing_surface_matrix 192); clippy clean.

src/dsl/parser.rs

@@ -315,12 +315,54 @@ fn is_select_projection_start(expected: &[crate::dsl::walker::outcome::Expectati
     has_word("distinct") && has_word("all")
 }
 
+/// ADR-0042 §3: detect the `… CROSS JOIN <table> on …` mistake. A
+/// CROSS JOIN takes no `ON` clause; the grammar rejects the `on`,
+/// but the bare structural error ("expected end of input") doesn't
+/// teach why. `on` is unexpected at this position *only* when the
+/// most recent join is a CROSS join — every other join flavour
+/// requires `on`, so there `on` would be in the expected set, not a
+/// failure. Detection: the failing token is the keyword `on`, and
+/// the last `join` word in the consumed prefix is immediately
+/// preceded by `cross`. Render-only; no grammar change.
+fn is_cross_join_on(source: &str, position: usize) -> bool {
+    let rest = source[position.min(source.len())..].trim_start();
+    let next_is_on = {
+        let mut chars = rest.chars();
+        let starts_on = rest.len() >= 2 && rest[..2].eq_ignore_ascii_case("on");
+        let boundary = chars
+            .nth(2)
+            .is_none_or(|c| !c.is_ascii_alphanumeric() && c != '_');
+        starts_on && boundary
+    };
+    if !next_is_on {
+        return false;
+    }
+    let consumed = &source[..position.min(source.len())];
+    let words: Vec<&str> = consumed
+        .split(|c: char| !c.is_ascii_alphanumeric() && c != '_')
+        .filter(|w| !w.is_empty())
+        .collect();
+    match words.iter().rposition(|w| w.eq_ignore_ascii_case("join")) {
+        Some(i) if i > 0 => words[i - 1].eq_ignore_ascii_case("cross"),
+        _ => false,
+    }
+}
+
 fn format_walker_error(
     source: &str,
     position: usize,
     at_eof: bool,
     expected: &[crate::dsl::walker::outcome::Expectation],
 ) -> String {
+    if is_cross_join_on(source, position) {
+        let consumed = source[..position.min(source.len())].trim_end();
+        let prefix = if consumed.is_empty() {
+            String::new()
+        } else {
+            format!("after `{consumed}`, ")
+        };
+        return format!("{prefix}{}", crate::t!("parse.cross_join_no_on"));
+    }
     let joined = if is_select_projection_start(expected) {
         crate::t!("parse.expect.select_projection")
     } else {