fix: warn on dropped builders and values; stop exporting EP internals

`h.event(1).team(["x"]).team(["y"]).ranking([0, 1]);` without the
terminal `.commit()` was a silent no-op: no warning, no error, and the
next thing the caller does is converge an empty history and read `None`
skills. `EventBuilder` already carried a `#[must_use]`; the value types
around it did not, so the same silence covered `Team::with_members`,
`Member::new`, `Outcome::*`, `Joint` and `Prediction::outcomes`.

`#[must_use]` now goes on the *types* rather than being sprinkled over
methods, which covers every constructor and builder setter at once and
gives the crate a rule where it previously had a list. Verified by
compiling a program that drops each one and reading the warnings back,
rather than by assuming the attribute took.

Visibility, from #73: `Gaussian::damp_natural` was reachable from
outside the crate despite being an EP damping internal called only from
`src/factor/`. The stray `pub fn`s inside the private `time_slice`,
`key_table` and `matrix` modules are now `pub(crate)`, so their
visibility states what it means instead of relying on the module being
private.

`storage/mod.rs` and `factor/mod.rs` become `storage.rs` and
`factor.rs`.

Closes #67. Refs #73.

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

src/event.rs

@@ -20,12 +20,12 @@ pub struct Event<T: Time, K> {
 
 /// A team: list of members competing together.
 #[derive(Clone, Debug, PartialEq)]
+#[must_use]
 pub struct Team<K> {
     pub members: SmallVec<[Member<K>; 4]>,
 }
 
 impl<K> Team<K> {
-    #[must_use]
     pub fn new() -> Self {
         Self {
             members: SmallVec::new(),
@@ -62,6 +62,7 @@ impl<K> Default for Team<K> {
 /// `InferenceError::ConflictingCompetitorConfig`: events in a batch have no
 /// order, so there would be no well-defined winner.
 #[derive(Clone, Debug, PartialEq)]
+#[must_use]
 pub struct Member<K> {
     pub key: K,
     pub weight: f64,