fix!: make the Time generic reachable

`History<T: Time, ..>` has always been generic over the time axis,
`Untimed` has always been exported, and `Drift<T>` is generic specifically
so that "seasonal or calendar-aware drift is expressible without going
through i64". None of it was reachable from a downstream crate.

Every construction route pinned `T = i64`: `History::builder()`,
`History::builder_with_key()`, and the only `Default` impl on
`HistoryBuilder`. Its fields are private and it had no `new`. So all three
escape routes failed to compile, and a consumer with domain timestamps
had to convert to i64 — which is the exact thing the parameter exists to
avoid. One of `History`'s four type parameters was paid for at every
signature and could never be varied.

`Default` is now generic over `T` and `K`, `HistoryBuilder::new()` exists,
and `time_type::<T2>()` / `key_type::<K2>()` join `drift` and `observer`
as type-changing setters:

    History::builder().time_type::<Untimed>().build()
    History::builder().key_type::<String>().build()
    HistoryBuilder::<Season, _, _, String>::new().build()

`key_type` replaces `builder_with_key`, which could not be turbofished —
`K` sat on the impl rather than the function, so callers had to spell
`History::<i64, _, _, String>::builder_with_key()`. 18 call sites across
15 files migrated.

tests/time_axis.rs is the part that matters. NOTHING in the repository
constructed a non-i64 history, which is precisely why this survived, so
the fix is only half done without a test that exercises the generic. It
defines a `Season(u16)` time type and a `SeasonalDrift` that accumulates
between seasons but not within one — the calendar-aware case the trait's
docs cite — and checks the whole path: fit, converge, and read a learning
curve whose times come back as `Season`, not as integers.

Two of the six tests are controls rather than assertions about output.
`Untimed` must ignore drift entirely, since elapsed is always zero, so
gamma 0.0 and gamma 5.0 must agree bit for bit. And a custom `Drift` must
actually widen a gap across seasons, or the test above would pass whether
or not the drift was consulted at all.

The README's ticked "Generalise a time axis" box is now true.

BREAKING CHANGE: `History::builder_with_key()` is removed. Use
`History::builder().key_type::<K>()`.

Closes #68

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

tests/determinism.rs

@@ -39,7 +39,8 @@ struct Fingerprint {
 }
 
 fn build_and_converge() -> Fingerprint {
-    let mut h = History::<i64, _, _, String>::builder_with_key()
+    let mut h = History::builder()
+        .key_type::<String>()
         .mu(25.0)
         .sigma(25.0 / 3.0)
         .beta(25.0 / 6.0)