explain: explain command end to end (ADR-0028 steps 2–3)

Add the `explain` prefix command — `explain show data`,
`explain update`, `explain delete` — from grammar through to a
rendered plan tree.

- Grammar: an `EXPLAIN` CommandNode whose shape is a Choice over
  the three explainable query shapes, referenced (not
  duplicated) through `Subgrammar`. `Command::Explain { query:
  Box<Self> }`; `build_show_data` is extracted so the role-based
  builders serve both standalone and explain-wrapped commands.
- Worker: SQL construction is split out of do_query_data /
  do_update / do_delete into `build_*_sql`, so EXPLAIN QUERY
  PLAN runs the exact same statement. `Request::ExplainPlan` /
  `do_explain_plan` capture the plan; `QueryPlan` / `ExplainRow`
  carry it back. EXPLAIN QUERY PLAN never executes, so
  explaining update/delete changes nothing.
- Display SQL: the executed statement with `?N` parameters
  inlined as standard-SQL literals via a quote-aware scan.
- Render: `render_explain_plan` draws the box-drawing plan tree
  (plain output; ADR-0028 step 4 adds the styled tree).
- Catalog: `parse.usage.explain` and the `help.data.explain`
  entry, so `explain` shows up in the in-app `help` listing.

1151 tests pass (+18); clippy clean.

src/friendly/strings/en-US.yaml

@@ -281,6 +281,10 @@ help:
         replay <path>              — run each non-blank, non-`#`-comment line of <path>
                                      as a command. Stops at the first error (no rollback);
                                      relative paths resolve under the project directory.
+    explain: |-
+        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)
   # Type reference, appended after the command list.
   types_reference: |
     Types: text, int, real, decimal, bool, date, datetime, blob, serial, shortid
@@ -440,6 +444,10 @@ parse:
     insert: "insert into <Table> [(<col>[, ...])] [values] (<value>[, ...])"
     update: "update <Table> set <col>=<value>[, ...] (where <col>=<value> | --all-rows)"
     delete: "delete from <Table> (where <col>=<value> | --all-rows)"
+    explain: |-
+      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)
     replay: "replay <path>  |  replay '<path with spaces>'"
     # App-lifecycle commands (per ADR-0003, surfaced through
     # the parser so they participate in usage templates +