feat: support explain over advanced-mode SQL queries

explain now wraps the advanced SQL commands — select, with (CTE),
insert, update, delete — in addition to the DSL show data/update/
delete it already covered, rendering through the same plan tree
(ADR-0039, closing the ADR-0030 OOS-2 gap).

Implemented as a second Advanced `explain` CommandNode under the
shared entry word, reusing the established shared-word dispatch
(SQL-first, DSL-fallback) rather than new grammar machinery.
build_explain_sql slices the inner SQL off the source and reuses the
existing SQL builders; do_explain_plan runs EXPLAIN QUERY PLAN over
the carried text verbatim (never executes, so safe for destructive
verbs). Advanced explain update/delete now route through SQL with an
identical plan; DSL-explain tests pinned to simple mode. Help and
usage text now list the advanced explain forms.

src/friendly/strings/en-US.yaml

@@ -321,6 +321,8 @@ help:
         explain show data <T> | explain update <T> ... | explain delete from <T> ...
                                    — show how the database would run a query, without
                                      running it (safe even for update / delete)
+        explain <select|with|insert|update|delete …>  (advanced mode)
+                                   — the same plan for the SQL you wrote
   # Type reference, appended after the command list.
   types_reference: |
     Types: text, int, real, decimal, bool, date, datetime, blob, serial, shortid
@@ -534,6 +536,7 @@ parse:
       explain show data <Table> [where <expr>] [limit <n>]
       explain update <Table> set <col>=<value>[, ...] (where <expr> | --all-rows)
       explain delete from <Table> (where <expr> | --all-rows)
+      explain <SQL select | with | insert | update | delete>   (advanced mode)
     replay: "replay <path>  |  replay '<path with spaces>'"
     # SQL `SELECT` (advanced mode; ADR-0030 / ADR-0031).
     select: "select (* | <expr>[ as <alias>][, ...]) from <Table> [where <expr>] [order by <expr>[ asc|desc][, ...]] [limit <n>]"