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/friendly/strings/en-US.yaml

@@ -500,6 +500,12 @@ parse:
   # completion/hints still expand the full first-set.
   expect:
     select_projection: "a projection: `*`, a column, or an expression"
+  # ADR-0042 §3: a CROSS JOIN pairs every row and takes no ON
+  # clause. The grammar rejects a following `on`; this message
+  # (rendered in place of the generic structural error when the
+  # most recent join is a CROSS join and the failing token is `on`)
+  # teaches the distinction instead of just "expected end of input".
+  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`"
   # Per-command usage templates (ADR-0021 §1). Rendered under a
   # "usage:" prefix when a parse fails after consuming a
   # known command-entry keyword. The bracket convention `[...]`