ADR-0021 implementation: per-command usage templates in parse errors

New `dsl::usage` module: registry pairing each command's
entry-keyword with a `parse.usage.*` catalog key.
`matched_entry()` resolves the entry keyword from the
consumed token prefix; multi-entry families (add, drop,
show) return all matching keys.

Catalog: new `parse.usage.<command>` keys (one per command),
`parse.token.{keyword,punct,...}` vocabulary (one per
Keyword/Punct variant + token-class labels + LexError
kinds), and `parse.available_commands` for the no-prefix
fallback. Catalog grows ~60 entries.

Validator: extended KEYS_AND_PLACEHOLDERS; new completeness
test asserts every Keyword and Punct variant has its
`parse.token.*` entry.

`app::dispatch_dsl` rewritten to compose three blocks per
ADR-0021 §2: caret + structural/custom error + usage block
(or available-commands fallback per §5). Caret math fixed
to use original-input byte position rather than
trimmed-input position (the lexer no longer trims before
lexing). Three pre-existing app tests adjusted to look
across all error lines instead of `output.back()` (the
usage block is now the last line).

`dsl::usage::matched_entry` uses `<=` rather than `<` for
position comparison so custom errors raised by `try_map`
(whose span starts at the first consumed token) still
resolve to the entry keyword.

Tests: 668 passing, 0 failing, 1 ignored (650 baseline →
+18: 8 usage + 1 token-vocab completeness + 9 new
integration tests in tests/parse_error_pedagogy.rs
covering create/add/drop/show/frobulate/update/insert
cases). Clippy clean.

src/friendly/strings/en-US.yaml

@@ -263,6 +263,95 @@ parse:
   # Default for the `ParseError::Empty` variant — surfaces as
   # `{detail}` inside the wrapper.
   empty: "empty input"
+  # No-prefix fallback (ADR-0021 §5): when the parse fails
+  # before any keyword is consumed, the renderer lists every
+  # command-entry keyword instead of attempting a per-command
+  # usage block. `{commands}` is an oxford-joined list of
+  # command-keyword renderings (each from
+  # `parse.token.keyword.*`).
+  available_commands: "available commands: {commands}"
+  # Per-command usage templates (ADR-0021 §1). Rendered under a
+  # "usage:" prefix when a parse fails after consuming a
+  # known command-entry keyword. The bracket convention `[...]`
+  # marks optional parts; angle-bracket `<...>` marks
+  # placeholders. ADR-0009's surface conventions apply.
+  usage:
+    create_table: "create table <Name> with pk [<col>:<type>[, ...]]"
+    drop_table: "drop table <Name>"
+    drop_column: "drop column [from] [table] <Table>: <Name>"
+    drop_relationship: |-
+      drop relationship <Name>
+      drop relationship from <Parent>.<col> to <Child>.<col>
+    add_column: "add column [to] [table] <Table>: <Name> (<Type>)"
+    add_relationship: |-
+      add 1:n relationship [as <Name>]
+        from <Parent>.<col> to <Child>.<col>
+        [on delete <action>] [on update <action>]
+        [--create-fk]
+    rename_column: "rename column [in] [table] <Table>: <Old> to <New>"
+    change_column: |-
+      change column [in] [table] <Table>: <Name> (<Type>)
+        [--force-conversion | --dont-convert]
+    show_data: "show data <Table>"
+    show_table: "show table <Table>"
+    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)"
+    replay: "replay <path>  |  replay '<path with spaces>'"
+  # Single-token vocabulary the renderer uses to translate
+  # chumsky's expected-set patterns. One key per Keyword variant
+  # (validated against `Keyword::ALL`), one per Punct variant,
+  # one per token-class label, one per LexError kind.
+  token:
+    keyword:
+      create: "`create`"
+      drop: "`drop`"
+      add: "`add`"
+      rename: "`rename`"
+      change: "`change`"
+      show: "`show`"
+      insert: "`insert`"
+      update: "`update`"
+      delete: "`delete`"
+      replay: "`replay`"
+      table: "`table`"
+      column: "`column`"
+      data: "`data`"
+      relationship: "`relationship`"
+      pk: "`pk`"
+      with: "`with`"
+      from: "`from`"
+      to: "`to`"
+      into: "`into`"
+      as: "`as`"
+      in: "`in`"
+      on: "`on`"
+      set: "`set`"
+      where: "`where`"
+      values: "`values`"
+      "null": "`null`"
+      "true": "`true`"
+      "false": "`false`"
+      cascade: "`cascade`"
+      restrict: "`restrict`"
+      action: "`action`"
+      "no": "`no`"
+    punct:
+      colon: "`:`"
+      open_paren: "`(`"
+      close_paren: "`)`"
+      comma: "`,`"
+      equals: "`=`"
+      dot: "`.`"
+    identifier: "identifier"
+    number: "number"
+    string_literal: "string literal"
+    flag: "flag (--name)"
+    end_of_input: "end of input"
+    error:
+      unterminated_string: "unterminated string literal"
+      unknown_char: "unrecognised character `{found}`"
+      bad_flag: "malformed flag (bare `--`)"
 
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