refactor: unify convergence defaults, validate builders, clear dead code

Convergence configuration had two disagreeing sources of truth and one
misleading report:

- `EpsilonOrMax::default()` capped at 10 iterations while
  `ConvergenceOptions::default()` allowed 30, and which applied depended on
  whether inference went through `run_chain` or a `Schedule`. The schedule
  default now derives from `ConvergenceOptions`.
- A graph with no iterating factors reported `converged: false` with an
  infinite step, despite being at its fixed point after the setup pass. It
  now reports converged with a zero step.
- `TimeSlice::iterate_to_convergence` hard-coded an epsilon and a
  20-iteration cap matching neither. It reads `self.convergence` and is
  scoped to `#[cfg(test)]`, which is all it was ever used by.

`HistoryBuilder::p_draw` and `::convergence` now validate their arguments
like `score_sigma` already did, instead of accepting a negative `p_draw` or
an `alpha` of zero — the latter leaves every EP update unapplied, so
inference silently returns the priors.

Removing the `#[allow(dead_code)]` masks let the compiler report what they
were hiding: four `OwnedGame` fields that were stored and never read, two
`ColorGroups` helpers and three `SkillStore` helpers used only by tests, and
`iterate_to_convergence` above. Test-only items are now `#[cfg(test)]` and
the unread fields are gone.

Also exported `HistoryBuilder`, which was public but unreachable — callers
could chain `History::builder()` but could not name the type — and added
`Rating::{prior, beta, drift}` and `Index::get`, so handles the API hands
out can be read back.

Two goldens moved, both convergence residuals rather than exact values:
`iterate_to_convergence` now runs to 30 iterations instead of 20, landing
nearer the symmetric truth of 25.0.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DnsaJg74eNSva3PJjK2eej

src/storage/skill_store.rs

@@ -41,6 +41,18 @@ impl SkillStore {
         }
     }
 
+    /// Whether a slot is occupied. Test-only.
+    #[cfg(test)]
+    pub fn contains(&self, idx: Index) -> bool {
+        idx.0 < self.present.len() && self.present[idx.0]
+    }
+
+    /// Number of occupied slots. Test-only.
+    #[cfg(test)]
+    pub fn len(&self) -> usize {
+        self.n_present
+    }
+
     pub fn get_mut(&mut self, idx: Index) -> Option<&mut Skill> {
         if idx.0 < self.present.len() && self.present[idx.0] {
             Some(&mut self.skills[idx.0])
@@ -49,21 +61,6 @@ impl SkillStore {
         }
     }
 
-    #[allow(dead_code)]
-    pub fn contains(&self, idx: Index) -> bool {
-        idx.0 < self.present.len() && self.present[idx.0]
-    }
-
-    #[allow(dead_code)]
-    pub fn len(&self) -> usize {
-        self.n_present
-    }
-
-    #[allow(dead_code)]
-    pub fn is_empty(&self) -> bool {
-        self.n_present == 0
-    }
-
     pub fn iter(&self) -> impl Iterator<Item = (Index, &Skill)> {
         self.present.iter().enumerate().filter_map(|(i, &p)| {
             if p {