MarketContext: coherent numeraire, day-count and calendar conventions

[marwinsteiner]

Context

The surface stores a flat r and per-slice forwards independently, and maturity is a bare float. Nothing enforces that F(T), D(T) and T come from one coherent convention, and T=0.5 does not say ACT/365F vs business/252. This is deeper than issue #13 (accepting curves): the invariant is that every price produced by a surface is consistent with one internally coherent discounting/forward/day-count convention.

Proposal

• MarketContext(discount_factor, forward, spot, dividend, day_count, calendar, valuation_time) as the single source of numeraire truth a surface is built against
• Date-aware API alongside year-fraction floats: expiry timestamps + day-count convention resolve to T internally
• Issue Term-structure rates and dividends for forward computation #13 (curves) should be designed as the first step toward this object rather than a standalone r-curve kwarg

Acceptance criteria

• A surface built from a MarketContext prices consistently (parity holds against the context discount factors by construction)
• Mixed-convention construction is impossible or loudly rejected
• Day-count choice demonstrably changes T, iv and theta in tests