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/config.rs

@@ -250,6 +250,8 @@ pub struct AppConfig {
     #[serde(default)]
     pub learn: LearnConfig,
     #[serde(default)]
+    pub compare: CompareConfig,
+    #[serde(default)]
     pub mcp_servers: Vec<McpServerConfig>,
     #[serde(default)]
     pub lsp_servers: Vec<LspServerConfig>,
@@ -323,6 +325,16 @@ impl Default for LearnConfig {
     }
 }
 
+/// Settings for the F7 compare screen — side-by-side generation with a
+/// test model against the current context.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct CompareConfig {
+    /// Backend name (looked up in `backends`) to use as the test model.
+    /// Empty = F7 reports "no test backend configured" and does nothing.
+    #[serde(default)]
+    pub test_backend: String,
+}
+
 fn default_user_name() -> String { "User".into() }
 fn default_assistant_name() -> String { "Assistant".into() }
 
@@ -340,6 +352,7 @@ impl Default for AppConfig {
             },
             dmn: DmnConfig { max_turns: 20 },
             learn: LearnConfig::default(),
+            compare: CompareConfig::default(),
             mcp_servers: Vec::new(),
             lsp_servers: Vec::new(),
         }