ADR-0022 stage 5/8: hint panel ambient typing assistance

ParseError::Invalid gains an `expected: Vec<String>` field —
the human-rendered names of the patterns chumsky was looking
for at the failure point (`\`create\``, `identifier`, etc.).
Empty for custom errors, which have no expected-set framing.
Populated by a new `describe_expected()` helper in parser.rs
that humanise() also delegates to (eliminates duplication).

`input_render::ambient_hint(input) -> Option<String>` returns
the hint-panel content per ADR-0022 §6:
  - empty input → None (caller falls back to panel.hint_empty);
  - Valid → t!("hint.ambient_complete") ("submit with Enter");
  - IncompleteAtEof → t!("hint.ambient_expected", expected = …)
    listing the parser's expected next tokens, oxford-joined;
  - DefiniteErrorAt → t!("hint.ambient_error_with_usage", …)
    composing the parse-error message with the matching
    parse.usage.* template if a known entry keyword was
    consumed, else the bare message.

Catalog gains the three hint.ambient_* keys + validator
declarations.

ui::render_hint_panel resolution order:
  1. explicit app.hint (modal contexts) wins;
  2. simple-mode + non-empty input → ambient_hint;
  3. fallback to panel.hint_empty.
Advanced mode (persistent + one-shot `:`) bypasses ambient
hinting per ADR-0022 §12.

Snapshot: highlighted_input_all_token_classes rebaselined
because the hint panel now displays an ambient hint instead
of the empty placeholder when input is non-empty.

Tests: 698 passing, 0 failing, 1 ignored (693 baseline →
+5 ambient_hint cases). Clippy clean.

Stage 6 introduces the IdentSlot taxonomy + parser audit so
identifier-typed slots can yield schema-aware completion
candidates in stage 8.

src/ui.rs

@@ -683,10 +683,26 @@ fn render_hint_panel(app: &App, theme: &Theme, frame: &mut Frame<'_>, area: Rect
         ))
         .style(Style::default().bg(theme.bg).fg(theme.fg));
 
+    // Resolution order for the hint panel body:
+    //   1. An explicit app-set hint (e.g. modal contexts) wins.
+    //   2. Otherwise, in simple mode with non-empty input,
+    //      the ambient typing-assistance hint (ADR-0022 §6).
+    //   3. Otherwise, the existing empty-state placeholder.
+    // Advanced mode skips ambient hinting (ADR-0022 §12) —
+    // the DSL lexer/parser don't speak SQL.
     let empty_hint = crate::t!("panel.hint_empty");
-    let body = app.hint.as_deref().unwrap_or(empty_hint.as_str());
+    let ambient = match app.effective_mode() {
+        EffectiveMode::Simple => crate::input_render::ambient_hint(&app.input),
+        EffectiveMode::AdvancedPersistent | EffectiveMode::AdvancedOneShot => None,
+    };
+    let body: String = app
+        .hint
+        .as_deref()
+        .map(ToString::to_string)
+        .or(ambient)
+        .unwrap_or(empty_hint);
     let paragraph = Paragraph::new(Line::from(Span::styled(
-        body.to_string(),
+        body,
         Style::default().fg(theme.muted),
     )))
     .block(block)