feat(routing)!: task rules become per-complexity model maps

The single per-task «Complexity» pin was the wrong shape — what you actually
decide is WHICH model serves a task at WHICH complexity. A task rule is now a
per-tier map, edited in the console as rows of “model ▾ + complexity ▾” pairs
(plus ×/add) inside each task:

  [routing.tasks.code]
  low  = ["cheap-coder"]
  high = ["best-coder"]
  any  = ["generic-coder"]   # leads at every tier after the tier pick

Chain order: the task's tier-specific list → (at high: the global high chain,
so escalation still beats any-tier pins) → the task's any-tier list → the
global tier chain → default as the last resort; deduplicated. Plain-array
rules keep meaning "any tier" unchanged, and the short-lived {models, tier}
form from the previous commit is normalized into the map on load.

Verified live: legacy config migrated visibly in the editor, pairs survive
save+reload, a math prompt at low ignored the math@high pair, per-tier leads
confirmed by unit tests (19 passing).

src/config.rs

@@ -408,38 +408,110 @@ pub struct RoutingCfg {
 
 /// A routing tier is either a list of model_ids or the string `default`.
 /// (`default` is handled by a separate field; only tier lists appear here.)
-/// Task rules additionally accept the full form
-/// `{ models = [...], tier = "high" }`, where `tier` FORCES the complexity
-/// tier for that task (e.g. legal review is always high, no matter what the
-/// router scored). Untagged: the map variant is tried first, so the old
-/// plain-array form keeps parsing unchanged.
+///
+/// TASK rules additionally accept a per-complexity map — "which model serves
+/// this task at each tier":
+///
+/// ```toml
+/// [routing.tasks.code]
+/// low    = ["cheap-coder"]
+/// high   = ["big-model"]
+/// any    = ["fallback"]        # leads at every tier after the tier-specific list
+/// ```
+///
+/// The plain-array form stays valid and means "leads at any tier". The
+/// transitional `{ models = [...], tier = "high" }` form is normalized into
+/// the map on load. Untagged: map is tried first, arrays keep parsing as is.
 #[derive(Clone, Debug, Serialize, Deserialize)]
 #[serde(untagged)]
 pub enum TierTargets {
-    Rule {
-        #[serde(default)]
-        models: Vec<String>,
-        #[serde(default, skip_serializing_if = "Option::is_none")]
-        tier: Option<String>,
-    },
+    Map(HashMap<String, TaskTierVal>),
     List(Vec<String>),
     One(String),
 }
 
+/// A value inside a task's per-tier map: a model list, a single model, or the
+/// transitional string value of the legacy `tier = "high"` key.
+#[derive(Clone, Debug, Serialize, Deserialize)]
+#[serde(untagged)]
+pub enum TaskTierVal {
+    List(Vec<String>),
+    Str(String),
+}
+
+impl TaskTierVal {
+    fn list(&self) -> Vec<String> {
+        match self {
+            TaskTierVal::List(v) => v.clone(),
+            TaskTierVal::Str(s) => vec![s.clone()],
+        }
+    }
+}
+
+pub const TASK_TIER_KEYS: [&str; 4] = ["any", "low", "medium", "high"];
+
 impl TierTargets {
-    /// The ordered model list, whatever the syntactic form.
+    /// Every referenced model id, whatever the syntactic form (validation).
     pub fn models(&self) -> Vec<String> {
         match self {
-            TierTargets::Rule { models, .. } => models.clone(),
+            TierTargets::Map(m) => {
+                let mut out = Vec::new();
+                for (k, v) in m {
+                    if k == "tier" {
+                        continue; // legacy marker, not a model list
+                    }
+                    out.extend(v.list());
+                }
+                out
+            }
             TierTargets::List(v) => v.clone(),
             TierTargets::One(s) => vec![s.clone()],
         }
     }
-    /// Forced complexity tier (task rules only).
-    pub fn forced_tier(&self) -> Option<&str> {
+
+    /// Normalized task rule: `(any_tier_models, per_tier_models)`.
+    /// Legacy `{models, tier}` becomes `per[tier] = models`; a plain array
+    /// becomes `any`.
+    pub fn task_rule(&self) -> (Vec<String>, HashMap<String, Vec<String>>) {
         match self {
-            TierTargets::Rule { tier, .. } => tier.as_deref(),
-            _ => None,
+            TierTargets::Map(m) => {
+                let mut any: Vec<String> = Vec::new();
+                let mut per: HashMap<String, Vec<String>> = HashMap::new();
+                let legacy_tier = m.get("tier").and_then(|v| match v {
+                    TaskTierVal::Str(s) => Some(s.clone()),
+                    _ => None,
+                });
+                for (k, v) in m {
+                    match k.as_str() {
+                        "tier" => {}
+                        "any" | "models" => any.extend(v.list()),
+                        "low" | "medium" | "high" => {
+                            per.entry(k.clone()).or_default().extend(v.list())
+                        }
+                        _ => {}
+                    }
+                }
+                if let Some(tier) = legacy_tier {
+                    // legacy pin: the models were meant for that tier
+                    let moved = std::mem::take(&mut any);
+                    per.entry(tier).or_default().extend(moved);
+                }
+                (any, per)
+            }
+            TierTargets::List(v) => (v.clone(), HashMap::new()),
+            TierTargets::One(s) => (vec![s.clone()], HashMap::new()),
+        }
+    }
+
+    /// Unknown keys inside a task map (validation).
+    pub fn invalid_task_keys(&self) -> Vec<String> {
+        match self {
+            TierTargets::Map(m) => m
+                .keys()
+                .filter(|k| !TASK_TIER_KEYS.contains(&k.as_str()) && *k != "models" && *k != "tier")
+                .cloned()
+                .collect(),
+            _ => Vec::new(),
         }
     }
 }
@@ -902,7 +974,7 @@ impl Config {
         }
         for (tier, targets) in &self.routing.tiers {
             let list = match targets {
-                TierTargets::Rule { models, .. } => models.clone(),
+                TierTargets::Map(_) => targets.models(),
                 TierTargets::List(v) => v.clone(),
                 TierTargets::One(s) => vec![s.clone()],
             };
@@ -926,14 +998,12 @@ impl Config {
                     );
                 }
             }
-            if let Some(tier) = targets.forced_tier() {
-                if !self.routing.tiers.contains_key(tier) {
-                    anyhow::bail!(
-                        "config: routing.tasks.{}.tier '{}' is not a defined routing tier",
-                        task,
-                        tier
-                    );
-                }
+            if let Some(bad) = targets.invalid_task_keys().first() {
+                anyhow::bail!(
+                    "config: routing.tasks.{}.{} is not a tier (use any/low/medium/high)",
+                    task,
+                    bad
+                );
             }
         }
         Ok(())
@@ -1026,7 +1096,7 @@ min_class = { low = "cheap", medium = "mid" }
 
         let low = back.routing.tiers.get("low").expect("low tier present");
         match low {
-            TierTargets::Rule { models, .. } => assert_eq!(models, &vec!["local-qwen".to_string()]),
+            TierTargets::Map(_) => assert_eq!(low.models(), vec!["local-qwen".to_string()]),
             TierTargets::List(v) => assert_eq!(v, &vec!["local-qwen".to_string()]),
             TierTargets::One(s) => assert_eq!(s, "local-qwen"),
         }