feat(schedule): add coupon/pillar schedule generation (Phase 0)

Closes Phase 0 foundations. Composes Date, Period, Calendar, and
BusinessDayConvention into accrual-period date sequences.

- Schedule + ScheduleBuilder: effective/termination/tenor + calendar +
  conventions; dates(), len, effective/termination_date, Index, iter.
- Drift-free generation from the anchor (anchor +/- i*tenor), then
  business-day adjusted and consecutive-duplicate deduped.
- DateGeneration: Backward, Forward, Zero, ThirdWednesday (IMM snap).
- StubConvention: ShortFront/LongFront (backward), ShortBack/LongBack
  (forward); natural short stub by default.
- end_of_month rolling; termination gets its own convention.
- third_wednesday(year, month) IMM helper.
- ScheduleError (#[non_exhaustive]): EmptyRange, NonPositiveTenor,
  StubDirectionMismatch.

14 new tests (121 total): even/short/long stubs both directions, EOM
month-end rolling, IMM snap, adjustment lands on business days, error
cases, iteration. fmt/clippy/doc clean.

Author: mqmalagris

src/lib.rs

@@ -71,6 +71,7 @@ pub mod daycount;
 pub mod linear;
 pub mod nelson_siegel;
 pub mod pchip;
+pub mod schedule;
 pub mod spline;
 pub mod svensson;
 
@@ -88,6 +89,7 @@ pub use error::YieldCurveError;
 pub use linear::LinearCurve;
 pub use nelson_siegel::NelsonSiegelCurve;
 pub use pchip::PchipCurve;
+pub use schedule::{third_wednesday, DateGeneration, Schedule, ScheduleError, StubConvention};
 pub use spline::CubicSplineCurve;
 pub use svensson::SvenssonCurve;