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feat(floatext): add `lerp_monotonic` to `FloatExt` (#866)

# Objective

- Reintroduce `FloatExt::lerp_monotonic`, which was removed from
`FloatExt` in #865 because adding a required method to the public trait
was breaking at the time.
- `FloatExt` (and the swizzle traits) are now sealed (#891), so the
trait can gain methods without breaking downstream implementors.

## Solution

- Add `FloatExt::lerp_monotonic` using the monotone form `self + (rhs -
self) * s` for `f32` and `f64`.
- Use it in `Vec3`/`Vec3A`/`DVec3` `slerp` length interpolation,
replacing the inlined monotone expression. Behavior is unchanged
(identical expression).
- Document the tradeoff versus `lerp`: monotonic in `s` and preserves
equal inputs exactly, but `rhs - self` is evaluated first, so it is less
accurate when the inputs differ greatly in magnitude and overflows to
infinity for opposite-sign extremes.
- Tests: `tests/float.rs::test_lerp_monotonic` covers endpoints,
extrapolation, exactness for equal inputs, and the documented overflow
behavior.

## Checklist

- [x] `cargo run -p ci` passes.
- [x] PR title follows [Conventional
Commits](CONTRIBUTING.md#pull-request-titles); mark breaking changes
with `!`.

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