fix: INSERT Form B value-count UX (ADR-0033 Amendment 5)

Three layered fixes for advanced/simple-mode positional INSERT
value-count mismatches (e.g. `insert into T values (...)` with the
wrong number of values for T's column count), plus ADR-0033
Amendment 5 recording the gate refinement.

Walker diagnostic (dml_insert_arity_diagnostics): the function's own
doc-comment recorded the no-column-list (Form B) case as deferred.
This commit closes that gap. Form B mismatches now emit a new
diagnostic.insert_arity_mismatch_form_b ERROR per offending tuple,
keyed off the target table's column count from the schema cache. The
[ERR] validity indicator (ADR-0027) lights up at typing time for the
reported scenario, no longer needing a submit.

Cross-mode pointer gate (advanced_alternative_note): refactored from
a hand-rolled Form B count check to a single input_verdict_in_mode(
input, schema, Mode::Advanced) call. The pointer fires only when the
verdict is None — the ADR-0027 sense of "valid". Any future static
check added to the verdict pipeline participates automatically; no
per-feature maintenance.

Teaching notes for the value-count cases users can hit before the
indicator turns red:
  - simple-mode submit: insert.form_b_extra_values_note covers under-,
    in-window, and over-supply against the Form B contract; suppressed
    when the cross-mode pointer fires (to avoid parallel advice).
  - advanced-mode dispatch pre-flight:
    insert.form_b_positional_count_mismatch_note catches a submitted
    mismatch with a teaching message before the engine produces its
    raw NOT-NULL / type error.

The advanced_mode.also_valid_sql pointer wording was reworked to
"trying to write SQL? switch with `mode advanced`, or prefix `:` to
run once". One insta snapshot regenerated.

ADR-0033 Amendment 5 records the gate change: Amendment 3's "would
parse in advanced mode" now reads as "valid in advanced mode" in the
explicit verdict-is-None sense, with the precise definition spelled
out so future readers can't drift back to the syntactic-only reading.
ADR-0000 index entry updated; docs/requirements.md H1a citation added
listing the three new pedagogical strings.

Tests added (8): four walker arity tests (under-supply, over-supply,
match, unknown-table); two app-level teaching-note tests for the
sibling cases (under-supply, over-supply beyond total); one pointer-
gate unit test pinning the bug-case suppression; one gate-precedence
test ensuring only one advice line per error. Existing
simple_mode_submit_of_sql_construct_appends_advanced_pointer updated
to use a known schema (the new validity gate requires it).

Full suite: 2031 passed, 0 failed, 0 unexpected skips. Clippy clean.

docs/requirements.md

@@ -526,7 +526,16 @@ handoff-14 cleanup; 449 after B2/C2.)
   INSERT removes one such case; ADR-0024's typed value slots
   give per-column-type rejection wording; `insert into T (col)`
   with no `values` clause now flags "looks like Form A — add
-  `values (...)`"). A systematic pass is still pending.
+  `values (...)`"; **issue #1 / ADR-0033 Amendment 5, 2026-05-28**
+  added two pedagogical lines that teach the INSERT Form B
+  positional-`VALUES` contract — a simple-mode submit-time
+  teaching note covering under-supply, over-supply, and extra
+  values (`insert.form_b_extra_values_note`) and an
+  advanced-mode dispatch-time pre-flight note for the same
+  value-count class (`insert.form_b_positional_count_mismatch_note`),
+  plus a walker-level `diagnostic.insert_arity_mismatch_form_b`
+  ERROR that lights the `[ERR]` validity indicator at typing
+  time). A systematic pass is still pending.
 - [ ] **H2** `hint` provides contextual help for the current
   input or the most recent error.
 - [ ] **H3** `help` provides general reference and per-command