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/outcome.rs

@@ -16,6 +16,7 @@ use smallvec::SmallVec;
 /// when `Some`; `None` inherits the history default.
 #[derive(Clone, Debug, PartialEq)]
 #[non_exhaustive]
+#[must_use]
 pub enum Outcome {
     Ranked(SmallVec<[u32; 4]>),
     #[non_exhaustive]
@@ -46,7 +47,6 @@ impl Outcome {
     /// `p_draw > 0`. Asking "team 5 won" and silently getting "everyone drew"
     /// is exactly the class of quiet wrong answer this crate keeps removing, so
     /// the check happens here where the mistake is.
-    #[must_use]
     pub fn winner(winner: u32, n: u32) -> Self {
         Self::try_winner(winner, n)
             .unwrap_or_else(|_| panic!("winner index {winner} out of range 0..{n}"))
@@ -73,7 +73,6 @@ impl Outcome {
     }
 
     /// All `n` teams tied.
-    #[must_use]
     pub fn draw(n: u32) -> Self {
         Self::Ranked(SmallVec::from_vec(vec![0; n as usize]))
     }