user: F7 compare screen

Side-by-side model comparison against the current conversation context.
Built on the MindTriggered pattern — F7 drops in as one more
CompareScoring flow next to MemoryScoring / FinetuneScoring.

Motivation: we have the VRAM on the b200 to load two versions of the
same family simultaneously (e.g. Qwen3.5 27B bf16 and q8_k_xl). Rather
than trust perplexity/KLD numbers on a generic corpus, we can measure
divergence on our actual conversations: for each assistant response,
ask the test model what it would have said given the same prefix, and
eyeball the diffs.

 - config.compare.test_backend — names an entry in the existing
   backends map to use as the test model. Empty = F7 reports "(unset)"
   and does nothing.

 - subconscious::compare::{score_compare_candidates, CompareCandidate,
   CompareScoringStats, CompareScoring}. For each assistant response,
   gen_continuation runs with the test client against the same prefix
   the original response saw; pairs stream into
   shared.compare_candidates as they complete.

 - user::compare::CompareScreen — F7 in the screen list. c/Enter
   triggers a run; list/detail layout mirroring F6, detail shows
   prior context / original / test-model alternate.

No persistence yet — each F7 run regenerates. Caching via a context
manifest (so we can re-view without re-burning generation) is the
natural follow-up; for now light usage is fine.

Also reusable later for validating finetune checkpoints: same pattern,
swap the test backend for the new checkpoint, watch where it diverges
from the base.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/mod.rs

@@ -4,6 +4,7 @@
 // machine, DMN, identity) lives in mind/.
 
 pub(crate) mod chat;
+pub(crate) mod compare;
 mod context;
 pub(crate) mod learn;
 pub(crate) mod scroll_pane;
@@ -64,6 +65,13 @@ fn screen_legend() -> String {
     SCREEN_LEGEND.get().cloned().unwrap_or_default()
 }
 
+/// Return the first line of `s`, truncated to `max` chars with an
+/// ellipsis suffix. Used by candidate-list screens.
+fn truncate(s: &str, max: usize) -> String {
+    let first = s.lines().next().unwrap_or("");
+    if first.len() > max { format!("{}...", &first[..max]) } else { first.to_string() }
+}
+
 /// A screen that can draw itself and handle input.
 trait ScreenView: Send {
     fn tick(&mut self, frame: &mut ratatui::Frame, area: ratatui::layout::Rect,
@@ -114,6 +122,8 @@ struct App {
     idle_info: Option<IdleInfo>,
     /// Fine-tuning candidates pending review.
     finetune_candidates: Vec<learn::FinetuneCandidate>,
+    /// F7 compare candidates — response pairs from test-model comparison.
+    compare_candidates: Vec<compare::CompareCandidate>,
 }
 
 impl App {
@@ -144,6 +154,7 @@ impl App {
             walked_count: 0,
             channel_status: Vec::new(), idle_info: None,
             finetune_candidates: Vec::new(),
+            compare_candidates: Vec::new(),
         }
     }
 
@@ -372,7 +383,7 @@ async fn run(
     }
     let notify_rx = crate::thalamus::channels::subscribe_all();
 
-    // F1=chat, F2=conscious, F3=subconscious, F4=unconscious, F5=thalamus, F6=learn
+    // F1=chat, F2=conscious, F3=subconscious, F4=unconscious, F5=thalamus, F6=learn, F7=compare
     let mut screens: Vec<Box<dyn tui::ScreenView>> = vec![
         Box::new(crate::user::chat::InteractScreen::new(
             mind.agent.clone(), mind.shared.clone(), mind_tx.clone(),
@@ -382,6 +393,7 @@ async fn run(
         Box::new(crate::user::unconscious::UnconsciousScreen::new()),
         Box::new(crate::user::thalamus::ThalamusScreen::new()),
         Box::new(crate::user::learn::LearnScreen::new(mind_tx.clone())),
+        Box::new(crate::user::compare::CompareScreen::new(mind_tx.clone())),
     ];
     let mut active_screen: usize = 1; // F-key number
     tui::set_screen_legend(tui::screen_legend_from(&*screens));
@@ -505,6 +517,9 @@ async fn run(
                 });
             }
 
+            // Sync compare candidates — a fresh run clears, so take a snapshot.
+            app.compare_candidates = ms.compare_candidates.clone();
+
             app.mind_state = Some(ms.clone());
         }
         app.walked_count = mind.subconscious_walked().await.len();