ADR-0019 implementation: friendly error layer + i18n catalog

All eight implementation steps from ADR-0019's §"Order of
operations":

Step 1 — `src/friendly/` module skeleton; `t!()` macro; YAML
  catalog loader (`include_str!` + `serde_yml`); `{name}`
  substitution helper that rejects format specifiers per §8.4.

Step 2 — `error.*` catalog populated for UNIQUE / FK /
  NOT NULL / CHECK / type-mismatch / not_found / already_exists /
  generic / invalid_value, with verbose hints per
  pedagogical-voice rule (§5). Anchor phrases (§10) preserved
  verbatim.

Step 3 — `FriendlyError { headline, hint, diagnostic_table }`
  + renderer composing the three blocks per §7.

Step 4 — `translate(&DbError, &TranslateContext) → FriendlyError`.
  Classifies by `SqliteErrorKind` first, then by message text
  for the constraint family. `change column` failures route to
  the type-mismatch headline, subsuming the previous
  `friendly_change_column_engine_error` helper.

Step 5 — `DbError::friendly_message()` delegates to the
  translator with default context. Removed
  `friendly_change_column_engine_error` (absorbed) and
  `enrich_fk_message` (FK list moves to the deferred re-query
  step). One test rewritten to assert on the engine-classified
  payload rather than the removed enrichment text.

Step 6 — `messages (short|verbose)` app-level command parallel
  to `mode`. `App::messages_verbosity` (default verbose)
  threaded into `TranslateContext` via
  `App::build_translate_context`. `AppEvent::DslFailed` now
  carries the structured `DbError`, plus the App extracts the
  user's attempted value from `Command::Insert` / `Update`
  to fill the `{value}` placeholder for UNIQUE / NOT NULL.

Step 7 — Catalog validator (§8.6) checks for missing keys,
  unused/undeclared placeholders, format specifiers, and
  forbidden engine vocabulary. `main.rs` parses the embedded
  catalog at startup so a corrupted build artefact fails
  loudly there rather than at the first `t!()` call.

Step 8 — Anchor phrases (§10) held: existing tests asserting
  on "no such table", "already exists", "cannot be converted",
  etc. all pass without rewording.

## Tally

603 tests passing (was 561: +42 net). Clippy clean with
nursery lints. Release binary 7.7 MB.

## Deliberately deferred

- Schema-aware enrichment for FK violations (parent_table /
  parent_column / child_table) and the multi-value
  natural-order INSERT case for UNIQUE. Both need the
  Database handle in scope at translation time, so they
  bundle naturally with the row-pinpoint re-query work
  (ADR-0019 §6) — that follow-on adds runtime-side
  enrichment via a `Database` lookup and a structured
  failure-context carried on `DslFailed`. Until then,
  unfilled placeholders render as their `{name}` form for
  visual consistency with the catalog.
- Migration sweep (§9). Only `error.*` is catalog-driven so
  far; `help.*`, `ok.*`, `client_side.*`, `replay.*`,
  `parse.*`, modal labels, etc. migrate per-PR.
- Settings persistence for `messages`. In-session state for
  now; waits on the future settings ADR.

src/event.rs

@@ -8,8 +8,8 @@
 use crossterm::event::KeyEvent;
 
 use crate::db::{
-    AddColumnResult, ChangeColumnTypeResult, DataResult, DeleteResult, InsertResult,
-    TableDescription, UpdateResult,
+    AddColumnResult, ChangeColumnTypeResult, DataResult, DbError, DeleteResult,
+    InsertResult, TableDescription, UpdateResult,
 };
 use crate::dsl::Command;
 
@@ -56,11 +56,13 @@ pub enum AppEvent {
         command: Command,
         result: AddColumnResult,
     },
-    /// A DSL command failed. `error` is already a friendly
-    /// message produced via `DbError::friendly_message`.
+    /// A DSL command failed. `error` is the structured
+    /// payload — App applies its current verbosity setting
+    /// (`messages_verbosity`) when rendering through
+    /// `friendly::translate_error` (ADR-0019 §5).
     DslFailed {
         command: Command,
-        error: String,
+        error: DbError,
     },
     /// Refreshed list of tables in the database.
     TablesRefreshed(Vec<String>),