Enforce required today's fixing before overnight coupon projection - #3060

When Settings::enforcesTodaysHistoricFixings() is enabled, the compounding overnight coupon pricer can bypass a missing current-date fixing through its telescopic discount-factor shortcut. Check the required history before entering the today-fixing fallback, outside its catch block.

The regression covers missing, stored-null and present current-date fixings with enforcement on and off, full and telescopic schedules, lookback and observation shift. Present and optional-history cases are checked against independently enumerated overnight deposit products. A future-only coupon remains forecastable without today's fixing.

Validation against cf74c9f657150750dfbf00ed6fd4c0768bc5ce36:

• The same new-test executable fails 12 assertions with the baseline library and passes all 86 assertions with the candidate.
• The affected overnight-coupon suite passes all 46 cases and 249 assertions.
• The final unfiltered stock suite passes 1,715 cases and 619,823 assertions, with no failures or skipped cases.
• git diff --check and the test-message check pass.

Native builds used Linux x86_64, GCC 14, Boost 1.87 and C++17, shared/default configuration, unity builds, -O1 -g0 -DNDEBUG and warnings as errors. The changed production and stock-test units were rebuilt and linked with hash-verified unchanged baseline objects; this was not a fresh complete rebuild.

This change is limited to CompoundingOvernightIndexedCouponPricer. Arithmetic averaging and the separate lockout and daily-spread corrections are outside its scope.