claude@clouddev1
d17addddd7
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.