feat: V5a show relationship/index <name> detail views

Fold the singular per-item forms into Command::ShowList { kind,
name: Option<String> } (name: Some = one item). Two grammar
branches reuse the relationship/index completion sources; worker
do_show_one renders a labelled detail block or a friendly
"No ... named X." line, reusing the V5 render path. Help +
parse-usage entries, two ADR-0042 near-miss rows, 5 integration
tests. Mark V5a [x] — V5's [<name>] clause now complete.

src/friendly/keys.rs

@@ -311,6 +311,8 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("parse.usage.show_tables", &[]),
     ("parse.usage.show_relationships", &[]),
     ("parse.usage.show_indexes", &[]),
+    ("parse.usage.show_relationship", &[]),
+    ("parse.usage.show_index", &[]),
     ("parse.usage.update", &[]),
     ("parse.usage.with", &[]),
     ("parse.expect.select_projection", &[]),

src/friendly/strings/en-US.yaml

@@ -325,6 +325,8 @@ help:
         show tables                — list all tables
         show relationships         — list all relationships
         show indexes               — list all indexes
+        show relationship <name>   — show one relationship's detail
+        show index <name>          — show one index's detail
     insert: |-
         insert into <T> [(cols)] [values] (vals)        — add a row
     update: |-
@@ -565,6 +567,8 @@ parse:
     show_tables: "show tables"
     show_relationships: "show relationships"
     show_indexes: "show indexes"
+    show_relationship: "show relationship <name>"
+    show_index: "show index <name>"
     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)"