fix: X4 — advanced-mode SQL INSERT auto-fills omitted non-PK serial (MAX+1)

A Form-A advanced-mode INSERT that omitted a non-PK serial column left it
silently NULL (the column is INTEGER UNIQUE, not NOT NULL, so SQLite
permits it), while simple-mode do_insert auto-fills it with MAX+1. That
violated ADR-0018 §1's "auto-generated on every path" contract and was the
unprincipled serial-vs-shortid asymmetry the ADR set out to remove
(advanced mode already auto-fills shortid).

Fix (decision: advanced mode matches simple mode): the advanced-mode
auto-fill reconstruction — renamed plan_shortid_autofill →
plan_autogen_autofill — now also fills an omitted non-PK serial with
MAX(col)+1 … MAX+n per row (single- and multi-row), reading MAX once under
the worker's single-writer serialisation. PK serial stays on the rowid
alias; Form B (no column list) still supplies every column. Honours
ADR-0018 §1/§5; no ADR amendment needed (the contract already said "every
path"). requirements.md X4 marked resolved.

Tests: 1949 passing (+1), 0 failed, 0 skipped, 1 ignored; clippy clean.

src/dsl/grammar/sql_insert.rs

@@ -105,7 +105,7 @@ fn fallback_value_list() -> Node {
 ///     `VALUES` position, so it is correctly absent here.
 ///   - **Form B** (no column list): ALL columns in declaration order,
 ///     including `serial` / `shortid` — advanced-mode Form B auto-fills
-///     *nothing* (`plan_shortid_autofill` returns early on an empty
+///     *nothing* (`plan_autogen_autofill` returns early on an empty
 ///     column list), so the user supplies a value for every column.
 ///
 /// Empty when schemaless, the table is unknown, or a Form A list