refactor!: un-export six types that no caller could reach

`TimeSlice`, `EventKind`, `KeyTable`, `CompetitorStore`, `Competitor` and
the `storage` module were all public and none was obtainable from a
`History` — `time_slices`, `agents` and `keys` are all private or
`pub(crate)`. `TimeSlice` was the worst: `new`, `add_events`, `iteration`,
`get_composition` and `get_results` were `pub` on a type you could only
build standalone and never feed back into anything.

Their sole consumer outside `src/` was `benches/batch.rs`, so a benchmark
was dictating six public types. It is rewritten against the public API: a
single-slice history's `converge` calls exactly the same per-slice sweep,
so capping at one iteration measures the same code path.

`N01` had zero references in the entire repository, including inside the
crate; removed. `N00` and `N_INF` are EP identities (`Add` and `Mul`) and
are now `pub(crate)` — a user reaching for `N_INF` as "an unknown
competitor's prior" would get an improper distribution whose `mu()`
silently reports 0.0.

Adds the accessors their absence forced people around, from #70:
`competitors()`, `competitor_count()` and `event_count()` (`size` had no
accessor at all). Answering "who is best" previously meant materialising
every competitor's full smoothed curve to read the last point of each.

`KeyTable::keys` now iterates the dense reverse table rather than the
forward `HashMap`, so `competitors()` yields insertion order rather than
per-process hash order — the same hazard as #62, caught before it could
reach a caller building a standings table.

Two `CompetitorStore` methods (`is_empty`, `iter_mut`) had no callers
anywhere and are gone; four more are now `#[cfg(test)]`, which is what
they always were in practice.

Worth recording a mistake: I first deleted `get_composition`/`get_results`
on the strength of a "never used" warning, and the build broke — the
warning came from the plain-lib target, where `#[cfg(test)]` callers in
history.rs are not compiled. A dead-code warning from one target is not
evidence about the others.

BREAKING CHANGE: `TimeSlice`, `EventKind`, `KeyTable`, `CompetitorStore`,
`Competitor`, the `storage` module, `N01`, `N00` and `N_INF` are no longer
public.

Refs #73, #70

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hcFjNDmHXZF8URGLku5zZ

src/time_slice.rs

@@ -760,6 +760,8 @@ impl<T: Time> TimeSlice<T> {
         }
     }
 
+    /// Test-only: reads the slice's shape back for assertions.
+    #[cfg(test)]
     pub fn get_composition(&self) -> Vec<Vec<Vec<Index>>> {
         self.events
             .iter()
@@ -773,6 +775,8 @@ impl<T: Time> TimeSlice<T> {
             .collect::<Vec<_>>()
     }
 
+    /// Test-only: reads the slice's shape back for assertions.
+    #[cfg(test)]
     pub fn get_results(&self) -> Vec<Vec<f64>> {
         self.events
             .iter()
@@ -887,7 +891,7 @@ mod tests {
 
     use super::*;
     use crate::{
-        KeyTable, competitor::Competitor, drift::ConstantDrift, rating::Rating,
+        competitor::Competitor, drift::ConstantDrift, key_table::KeyTable, rating::Rating,
         storage::CompetitorStore,
     };