ADR-0022 stage 8 follow-up: fixes from real-app testing

Three fixes from the user's testing run, plus an
investigation note on a fourth.

#4 Sticky hint during cycling. The previous code recomputed
candidates_at_cursor at the post-Tab cursor position, which
made the panel whiplash through "what comes next at the new
cursor" between cycles. ambient_hint now short-circuits to
the memo's stored candidate list while the memo is alive —
so Tab Tab Tab keeps showing the same list with the
selection moving, then snaps to the post-Tab ambient state
once any non-Tab key clears the memo.

#2 Candidate ordering and kind-coloured rendering. New
`Candidate { text, kind: Keyword|Identifier }` carries the
classification through completion, last-completion memo,
and ambient-hint payload. candidates_at_cursor now sorts
keywords first (alphabetical), identifiers second
(alphabetical), and the hint-panel renderer colours keywords
in `tok_keyword` and identifiers in `tok_identifier`.
Keyword-vs-identifier name collisions resolve in favour of
the keyword (rare; the user can still address their table
via different syntax).

#3 tok_identifier no longer matches theme.fg. Identifiers
in the input pane now render in a distinct cool grey-blue
(dark) / dark steel-blue (light), so they stand out from
prose-like default text without competing with keyword
purple. Same colour drives the identifier candidates in
the hint panel for visual consistency input ↔ hint.

Limitation worth knowing: "keywords first, alphabetical"
is not the same as grammatical order. For "add column "
the hint shows `table to` not `to table` — chumsky's
expected-set doesn't preserve combinator-source order, and
encoding it in the registry adds maintenance overhead the
fix doesn't cleanly justify. Marked for future revisit if
it bites.

#1 (Tab does nothing on "add column ") — not reproduced
through App::update. The internal logic works correctly:
"add column " + Tab inserts "Customers ", second Tab
cycles to "Orders ", third to "Thing ". The most likely
explanation is a stale binary or a terminal-level event
intercept (tmux focus, kitty-keyboard protocol differences,
etc.) — needs user verification with a fresh build.

Tests: 747 passing, 0 failing, 1 ignored (744 baseline →
+3: 2 new completion-ordering cases including the
keyword-wins-on-name-collision edge, plus 1 hint-mid-cycle
sticky test). Clippy clean.

src/ui.rs

@@ -730,16 +730,20 @@ fn render_hint_panel(app: &App, theme: &Theme, frame: &mut Frame<'_>, area: Rect
 }
 
 /// Render the candidate-list line for the hint panel
-/// (ADR-0022 §7 + the user's #2). Items are space-separated;
-/// the selected item (if any) gets a brighter colour + bold;
-/// when the items overflow `width`, a window centred on the
-/// selected (or starting at item 0 with no selection) shows
-/// scroll markers `<` / `>` at the edges.
+/// (ADR-0022 §7 + the user's #2). Items are space-separated.
+/// Each candidate is colour-coded by kind — keywords in
+/// `tok_keyword`, identifiers in `tok_identifier` — so the
+/// user can tell command grammar apart from schema names at
+/// a glance (post-stage-8 user feedback). The selected item
+/// (if any) gets bolded; when the items overflow `width`,
+/// scroll markers `<` / `>` appear at the edges with the
+/// window centred on the selection (or item 0 with no
+/// selection).
 ///
 /// Returns `Line<'static>` (each item cloned into its span)
 /// so the caller doesn't have to manage the items' lifetime.
 fn render_candidate_line(
-    items: &[String],
+    items: &[crate::completion::Candidate],
     selected: Option<usize>,
     width: usize,
     theme: &Theme,
@@ -747,29 +751,32 @@ fn render_candidate_line(
     if items.is_empty() {
         return Line::default();
     }
-    let item_style = Style::default().fg(theme.muted);
-    let selected_style = Style::default()
-        .fg(theme.fg)
-        .add_modifier(Modifier::BOLD);
+    let separator_style = Style::default().fg(theme.muted);
     let marker_style = Style::default().fg(theme.fg);
+    let style_for = |i: usize| {
+        let base_fg = match items[i].kind {
+            crate::completion::CandidateKind::Keyword => theme.tok_keyword,
+            crate::completion::CandidateKind::Identifier => theme.tok_identifier,
+        };
+        let mut s = Style::default().fg(base_fg);
+        if Some(i) == selected {
+            s = s.add_modifier(Modifier::BOLD);
+        }
+        s
+    };
 
     let total_width: usize = items
         .iter()
-        .map(|s| s.len() + 1)
+        .map(|c| c.text.len() + 1)
         .sum::<usize>()
         .saturating_sub(1);
     if total_width <= width {
         let mut spans: Vec<Span<'static>> = Vec::with_capacity(items.len() * 2);
         for (i, item) in items.iter().enumerate() {
             if i > 0 {
-                spans.push(Span::styled(" ".to_string(), item_style));
+                spans.push(Span::styled(" ".to_string(), separator_style));
             }
-            let style = if Some(i) == selected {
-                selected_style
-            } else {
-                item_style
-            };
-            spans.push(Span::styled(item.clone(), style));
+            spans.push(Span::styled(item.text.clone(), style_for(i)));
         }
         return Line::from(spans);
     }
@@ -780,12 +787,11 @@ fn render_candidate_line(
     let center = selected.unwrap_or(0);
     let mut left = center;
     let mut right = center;
-    let mut used = items[center].len();
+    let mut used = items[center].text.len();
     let avail = width.saturating_sub(4);
     while (left > 0 || right + 1 < items.len()) && used < avail {
-        // Expand right first, then left.
         if right + 1 < items.len() {
-            let cost = items[right + 1].len() + 1;
+            let cost = items[right + 1].text.len() + 1;
             if used + cost <= avail {
                 right += 1;
                 used += cost;
@@ -793,7 +799,7 @@ fn render_candidate_line(
             }
         }
         if left > 0 {
-            let cost = items[left - 1].len() + 1;
+            let cost = items[left - 1].text.len() + 1;
             if used + cost <= avail {
                 left -= 1;
                 used += cost;
@@ -811,14 +817,9 @@ fn render_candidate_line(
     }
     for (offset, item) in items[left..=right].iter().enumerate() {
         if offset > 0 {
-            spans.push(Span::styled(" ".to_string(), item_style));
+            spans.push(Span::styled(" ".to_string(), separator_style));
         }
-        let style = if Some(left + offset) == selected {
-            selected_style
-        } else {
-            item_style
-        };
-        spans.push(Span::styled(item.clone(), style));
+        spans.push(Span::styled(item.text.clone(), style_for(left + offset)));
     }
     if need_right_marker {
         spans.push(Span::styled(" >".to_string(), marker_style));